Here's the thing. I have very, very limited time and I don't even had time to prepare everything for my adventures, let alone the rest of the game world. If you do, great. Now, I'm very good at improvisation, so it doesn't bother me to make something up on the fly. However, many DMs are not good at improvisation and need those details figured out in advance.
I generally only have time to create an outline for my adventures with some encounters and have to improvise everything in-between. Were I one of the many DMs who wasn't good at improvisation, I wouldn't be able to DM a game that wasn't pre-made. Having an updated version of Greyhawk with those details filled in by WotC would allow those DMs to run Greyhawk.
You can't just expect those sorts of DMs to just "make a decision" when it comes up. Creativity is not the same as making a ruling on a rule.
I've been running my own home campaign for decades. I've never figured out all the details ahead of time. I simply have a list of random names sorted by race and gender that (nowadays) I get off the internet.
But I don't understand how anyone would run an ongoing campaign unless rely solely on published mods. The moment you go off the rails in my game, you have the possibility of changing the world. The Purple Pony? Yeah, that burned down after an idiot wizard cast fireball. The king? Well, unfortunately the PCs didn't stop that assassination attempt. And so on.
As far as details of taverns and the like, there are plenty of generators out there. But how much of that is in any source book to begin with?
Last, but not least, I simply don't care about details until they matter. Personally I find running a published mod much more time consuming.
But how to do a home campaign is a separate topic.