As an example, look at most of Linux Distro's UX since you brought it up. They aren't meant for the layman the way Mac/Windows was. the deeply technical audience, for better or worse, puts up with a lot of UX issues for their tools. That doesn't work the same way for a general audience website or app.
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But that's from the "why can't programmers art" side of the argument. Of course, that begs the question "why can't programmers find artists to work with them?"... well, there's a dozen different reasons. Culturally, historically, socially, and so on.
But to list the one big reason; there aren't stereotypes about programmers being short on cash and needing to commissions out programs after their full time job just to get by. (a few) programmers do all that just for their own self-fulfillment or for their own goals, with no expectations of a big payment most of the time. Because many are already financially comfortable.