The quintessential question concerning why the aliens do or don't do what they do or not ... is ...
This point of view implies that the president, et al, is somehow an important person of note who is the representative of all the people.
Of course, he is not.
The president (this one now and all those before him) simply represents himself ... his own views. He is not a genious or mastermind of objective reality. He's just the local "schmo" of the moment. His schtick is to engage others of his ilk in social fencing governed by arbitrary rules which often results in warfare to some degree. The fact that people are killed by the millions does not make his "work" any the less arbitrary. Because what he does is not objective, all that follows is just "whimsy" regardless of the ill effect generated. This goes for all of the earth's other multitudinous potentates and potentate-wannabees.
For a race of objectively oriented beings, they are no more significant that any other local "schmo". Hence, they might just as well land on his lawn as any other ... but not during the day.
Daytime appearances are not allowed
For the most part ...
It's OK to be seen but only in such a way that can be discounted upon reflection. That is, they can't appear by the Empire State building and be seen and photographed for minutes on end by thousands of people. That would be "undeniable" evidence. On the other hand, if they appeared so and behaved exactly like a balloon ... they might get away with it because the skeptics could say "There was no anomalous motion ... therefore ... though it was definitely a solid object ... it was probably an elaborate balloon hoax".
Hence, they have strict rules about how they are to be perceived by us. Seen but not Proved.
You must see our world from their temporal perspective as best you can. And that perspective is truly vast. Our galaxy may have been completely colonized for as much as four billion years. If you accept the Big Bang theory or some variant, i.e. a universe with a beginning ... then, to have intelligent life evolve, you must wait around several billions years for heavier elements to form out of which you can construct such beings. You can't make them out of just hydrogen and helium (probably can't).
Now, assuming that the first planet generates an intelligent life form, it would take something on the order of two million years to colonize every possible rock in the entire galaxy. This is generally conceded by scientists (the more reputable as well as the pundants) ... if ... travel between stellar systems is indeed possible. We are assuming here, of course, that it is possible. Even if restricted to sublight speeds, the answer is going to be roughtly the same ... something on the order of a few million years (2 million or 100 million even). Still this is nothing close to the already mentioned four BILLION years.
So, put yourself in their shoes. What do you do when you encounter a "rock" that is going to produce an indigenous life form? Should you waste it? (kill) ... and insert your own same 'ol, same 'ol ... or ... sit back and watch the struggle for life play out from afar? Certainly, the development of a new planet from scratch must be at least comparatively rare. I am assuming therefore that the general consensus among such spacefaring planets is to ... let it be.
Aliens never break ranks
They do all sorts of weird things ... but they never break the cardinal rule of always leaving their presence "deniable". To do otherwise is to interfere with the earth's development. They are as one on this issue. And what would they gain from interfering? We have nothing they need physically. By interfering, they would violate their own ethics ... developed over hundreds of millions of years. Whatever their ethics are ... they are very, very old indeed. And they have fully explored every conceivable alternative before settling on "the best way" all things being considered.
Now, suppose you think that they will one day contact us overtly and undeniably in broad daylight. I must then pose the question of ... when ... should they do so? Now? Just now? Why not one thousand years ago? Why not one thousand years from now? In short, when is there some unambiguous instant wherein all alien races involved would say, "Now is the time to go down there and show ourselves to these young puppies".
I see only one unambiguous moment when they can communicate with us as reasonable "equals". And this is at the point of our discovering them (on our own ... by our own means). I mean ... we go to their planet and find them there. Thus, if we have the ability to find them ... only then ... are we unambiguously worthy of the epithet ... spacefaring. Unfortunately, this means that as we come of age, in their estimation, they should probably leave without a word. Leave us alone completely for the first time in millions of years till such time as we appear on their doorstep ... maybe in a few thousand years more. Remember, if these people are real ... their perspective is vast and a few thousand years means nothing to them.
In the meantime, they exercize their own rights as sentient beings to explore our world for what amusement or interest it offers them ... at the same time honoring our right to develop independently. And they probably would like to see us stop killing one another, etc. but not so much as to interfere, which would be a bigger faux pas to their way of thinking.
In my opinion ... based upon what I have read about them or seen in documentaries ... they are fairly playful and like to poke holes in our "puffery" and pull people's legs (although I've never seen any report of an alien actually laughing). They seem to be taking genetic samples as well but I don't know if thie is symbolic, imagined or has some sort of purpose. Certainly, if they had any genetic goals, they could be achieved without so many supposed abductions. I would tend to guess that the whole genetic thing is more philosophical than physical. That is, if they are taking genetic material ... they simply don't need it. They have all that they could possibly need or want for any purpose whatever out of just a few people and this would have been done already ages ago.
There is one possible physical motive for their behavior
They may want to eat us.
Let us suppose that they like the gamey taste of natural human steak. Then, they may decide that the best course (pun) is to leave us be ... thinking that we are most likely to overbreed when happy and contented. Then, when they judge that the thing is ready, they swoop down all over the place (at night) and kidnap dinner. If there are billions of humans and they only eat a few million per year ... nobody notices. We just put their faces on milk cartoons and wonder where did little Johnny disappear to?
They abduct people periodically to see it the planet is "done", like poking a turkey in the oven. And maybe they like semen on their ice cream so they "milk" this or that guy's scrotals.
Isn't the above ridiculous? I think so. But that's the only alternative I can come up with that matches the known behavior of our aliens. Of course, if nobody is there ... there's no problem ... except the continued sightings ... every year ... by credible people ... of craft doing incredible things that nothing on earth can yet do ...Aliens as Gods