I have to agree with you that the themes on WordPress.org are the face of WordPress. Which makes it even more absurd the fact that in the back-end, when you open the Themes page, we still get those louzy tabs with "Featured / Popular / Latest".
The featured tab is first, and it's just a random list of (often terrible) themes.
At least rename it to "Random", so that new WordPress users don't think that this random collection of themes are somehow the best of what WP has to offer.

As for some of the most popular multipurpose themes: the keyword is affiliate marketing. If these themes have good marketing skills and a great sales funnel, content creators will promote them.
Why promote a niche shop with a 0.5% conversion rate when you can promote someone with a 2% conversion rate and a higher commission rate?
And once marketers get on with it, the smaller guys are screwed.

Just a sad reality of doing business.