and that is one of the quirks of Polish.
You will notice that declension tables for masculine nouns have two accusative forms, like czarny/czarnego.
This is because in Polish in masculine singular nouns divide between animated and not animated. Animated nouns have accusative= genitive; not animated nouns have accusative=nominative
Płaszcz has accusative=nominative, so the adjective describing it has to match.
A raincoat could be a płaszcz przeciwdeszczowy or a płaszcz nieprzemakalny, but when I did a google image search for just płaszcz, there weren't any raincoats there, whatsoever. Unlike its Russian cognate плащ the word alone isn't really being associated with a raincoat, apparently.
Well, yes, it is the same in English. Technically, I currently wear a 'raincoat'. (Not the plastic or rubber variety, closer to a trench coat.) However, I just call it my 'coat'. And then if asked 'which one?' , I would say "the raincoat".
I had asked here because duolingo gives 'raincoat' in the translations. I ended up getting it wrong not because of that particular translation but some other error I made and I misunderstood at the time.