I suspect there's a LOT of homebrew going on, but you never see it because nobody can really talk about it with anyone else. Running a published adventure lets you compare notes with other GMs and players who've done the same one. So that gets to be visible.
When I started writing my gaming-related blog a couple years ago, one of the things I decided to use it for was posting session summaries of my homebrew campaign (though I do that with Yawning Portal, too). That way, I can share what I'm doing with the setting with a wider audience than just my players. I also have a public wiki with world info for my players, and anyone else who's interested. And finally (though chronologically firstly), long before the campaign was ready to start play, I started discussing my ideas for it on a RPG forum (The Piazza) where a lot of other GMs share their homebrew worlds.
These days, the best 'mix' I can see is home-brewed adventures in established published worlds. The world is important for everyone to have a general idea of - deities, nationalities, and such - having good sources for that information available to everyone lets people settle into a character and adventure easier. Pre-written adventures, however, seem a little to railroady for me.