First of all major corporations contribute to Linux kernel and there is very little contribution to a distribution. Why are they doing it? Because they benefit from their hardware being supported by Linux kernel(e.g Samsung contributing to Linux Kernel for SSD drivers) and now they can sell more, they can do this because it works with their business model. That is not the case with smartphones, in the smartphone world they are selling directly to a consumer and they need to do everything they can to differentiate themselves from other Smartphone makers. Mozilla tried the business model you mentioned but it didn’t catch on. Lastly you forget to understand the number of apps available on Google Play vs on Flathub. Google Play has ~3.5 Million Apps vs ~2000 Apps on Flathub. We are talking a different scale here

Also speaking about Flathub, Flathub solves the issue of fragmentation by building an entire OS on top of another OS just to avoid the challenges of backwards compatibility. This has implications like huge app sizes because you are basically downloading the runtime and everything it depends on for each app. It works for most people because storage is cheap and can be upgraded at least in PC world. But still you will have issues with RAM because most flatpaks don’t share the runtime and you need to need load each runtime to memory and this implications like higher memory usage, slower app start times because you need to load the entire runtime first before even you start the app.