User Agents look the way they do because this is a recurring issue.
A browser without network effects gets blocked, they look for a way to bypass the blocking, then they become mainstream and now the de-facto UA is larger than before.
And use your brain for a hot second will you? Bad actors don't use a rare user agent, they use the same Chrome user agent that everyone else uses.
It's unfortunate yes but that's what drives the threat signatures
edit: it's noted downthread that automated testing of card details to find valid ones is a reason.
Demographic is important here. If I was running a shop that sold software for Linux users, sure. If I'm running a store that sells pretty much anything else? I'm not caring.
I'd think that a non-standard browser also strongly suggests that they're a financially-comfortable middle-class individual, and quite possibly a whale with FAANG income.
Sure. If there was another place to buy a better door at. But if that door manufacturer's the only one that makes doors, if the door installer and door technicians all tell you that they can't or won't make another door for you, then you just deal. Maybe crank up the prices a bit to try to mitigate your 10% shortfalls.
The place where a business looks at that problem and sees money being left on the table that it can't live without and that it has no other way of making up for... that is a very narrow stretch, and only very marginal businesses live there.