Same reason collectivist people like social media censorship and gun control, to make them feel “safe” even though all it does is centralize power. Besides hi ow many people have the tech skills to even know what third party app repositories are?

They exist. People just don’t buy them. But there is a Ubuntu phone port you can install on your phone as an alternative to android.

https://forums.ubports.com/topic/10762/status-update-on-the-next-noble-based-ubuntu-touch-release-february-2025

But yeah it can get complicated like any Linux community project and isn’t at all mainstream.