Because portability has only been practical for the majority of applications since 2005ish.
You're not having a system where every executable has 100mb of OS libs statically linked to them in the 90's be fuckin for real.
You complain a lot about static linking in rust and it's the only way to actually achieve portability.
My brother in Christ if you have to put every lib in the stack into a GUI executable you're gonna have 100mb of libs regardless of what system you're using.
Plan 9 was a centrally managed system without the speed of development of a modern OS. Yes they did it better because it was less complex to manage. Plan 9 doesn't have to cope with the fact that the FlatPak for your app needs lib features that don't come with your distro.
It was literally not practical to have every app be portable because of space constraints.