If you’re saying why we don’t have multiple app stores run by multiple people and even self hosted apps then I can help add an answer.
It was exactly like this before Android and iPhone. I had a Nokia E71 and before that a Sony Walkman phone. They could run J2ME apps and games and you could download from a site called getjar.com and others. Nokia Symbian could run more powerful native apps - I wrote one myself.
They all sucked. Most of the apps were full of malware and you never knew which one would lock up your phone, infect your phone, or upload all your data to somewhere.
AppStore and Playstore exist for the same reason that CA certificates exist - most people are good but some are terrible and they damage the ecosystem. For CA we have LetsEncrypt now. But phones are way more complex than that. Most of the phone stack is thankless but highly specialized work - you won’t get that for free in volume anytime soon.
It’s more complicated because people expect the latest phones. Older generation mobile technology is worthless right now. Cutting edge technology needs investment and has substantial market. This market is also winner takes all. There is maybe place for 3 players but not anymore in an efficient, commodity market that’s also capex heavy.