Saying "oh just do the thing you aren't doing" shows that one of two things is true: Either every app developer currently doesn't know the web exists, or everyone else knows something you don't.
On Android for example, Play Store does not have "all" apps, or even most apps that I have on my phone. Because Play Store has mostly garbage. It's a surprise to no one that Steam's storefront has a different selection from Amazon or Epic or GoG in the same way that no one is surprised that Walmart and Target carry different items.
So "discovery" in your sense is basically "I ignore everything that isn't presented by this particular storefront", i.e. a complete lack of discovery.
https://www.humblebundle.com/store/c/all?sort=bestselling&pa...
https://www.amazon.com/mobile-apps/b/?ie=UTF8&node=235014901...
https://play.google.com/store/
https://store.steampowered.com/ (yes, you can run PC games on Android if your hardware is up to the task)
https://store.epicgames.com/en-US/
Apple doesn't have some special magic "discoverability"; they just prevent any other stores from being able to compete with them. Actually, searching for the iOS app store, I get:
https://www.apple.com/app-store/
which has no obvious way to even browse or search apps? How am I supposed to see what's available? I need to buy an expensive phone before I can even see what it can run? This is more discoverable? The bottom nav bar entry for "App Store" just takes me to the same page. If I click on "Store" it just shows me hardware; no apps. Where's the store and how am I supposed to find it? Do I need to physically go into an Apple store and use a phone there to browse?
There are more app-having companies that have web apps than not
Of the mini-apps in China, I don't know, I was never exposed to the existence of their non-mini-app counterparts.