Art supplies were historically not cheap. If you wanted to do this for a living, you were probably needing to aim for selling your art to the rich upper class. That implicitly meant catering to their fickle tastes and working on commission. You didn’t make art for you and find your audience later, you made art for the customers you had or you starved.

And to put it bluntly, realism wasn’t the fashionable hotness for most of human history. The more “crude” styles you may think of as objectively inferior to and less technically impressive as realism were in fact the styles in demand at their respective times. Fashion existed in ancient and medeival times just like it does today, and those styles were the fashion.

The idea of the independent eccentric artist who lives secluded in their ideas cave producing masterpieces for no one in particular leaving the world in awe at their genius every time they come out with something to show is a very modern concept. If any artist wanted to make a realism painting in an era where it was not popular, they’d be doing it purely for themselves at their own expense. So virtually no one did. Or if they did, their works largely didn’t survive.