As an end user, I know knowing that stuff in Apple's App Store is reasonably vetted. It's not perfect, and when mistakes are made we ready about them here. But for the most part if I install an app that says it'll do X, it'll do X while generally not being to upload unrelated data to North Korea. I'm at a stage in life where I'd rather pay someone to make those investigations and decisions for me than invest the time to do it myself. Again, other people strongly feel otherwise, but that's OK!
As a family member, I have to worry a little less about what apps my kids are running, and have to spend a little less time fixing my older relatives' phones when I go home to visit. This gives me more time to do stuff I actually want to be doing.
As a security person, I have to worry a little less about what apps my coworkers are running, and I know there's a reasonably good sandbox keeping one app's data away from another's. If there were, say, a Facebook-encapsulated app store, I might have zero confidence that the game app they installed through it can't access data in the "add cat pics to your presentation!" app that they also installed through it.
I totally get why people want to have multiple app stores on their phone, and I can't say they're wrong. Still, I'm perfectly happy with the current state of affairs, and have no desire to have Epic and Facebook app stores on my kids' or coworkers' phones that I'm at least a little bit responsible for.