Because there aren't two kinds of shirt, striped or solid coloured. You even acknowledge that by mentioning Hawai'ian shirts. You might want a small check shirt, a mostly plain shirt but not mind logos so not completely monocolour, a plaid shirt, a large check lumberjack style, spotted or dotted or diamond pattern, flowery pattern, you might not even know what the other available options are to search for them one by one - you just know that you don't want stripes.

There is no ambiguity in "not stripes", you can't invert it and write it in the positive form of what you want; the neatest way to describe the category of what you want to browse is "things which are not stripey".

Particular personal bugbear is car websites where you can filter in "petrol engine" or "diesel engine", but there is no support for negative filtering, so you can't choose "not LPG". In so many search-and-filter options you can't exclude your dealbreakers, and it's much more likely that I have a single dealbreaker which rejects a choice overriding all other considerations, than that I have a single dealmaker which makes a choice overriding all else.