Garbage bags come on a roll. People have been using the big ones for raincoats for decades. so why don't raincoats come on a roll? ... Cheap disposable ones.
Today, I got caught without an umbrella so I had to make a kind of "babushka" out of a small plastic bag ... not the least bit elegant. I looked like a bubble-headed chef. I don't need a raincoat so much to keep dry as to keep from getting sick. That's the real goal here.
The trouble with using the regular garbage bag is that it has no head hole or no head covering. If you put your head through a hole torn in the top, you've nothing covering your head. If you stick your face into a small hole on the front of the bag your shoulders ought to be up in the air like Fred Munster. I want a stylish cut for my 10 cents ... dammit.
How about something like this? You just pull one off the roll like a bag and put it on. Then throw it out when the rain stops.