Comment by aragilar

One could ask why AOSP was created when there was an existing userland on linux that could be used (and was, by Nokia).

More seriously, I think the reason people want to do this is threefold: 1. Android vendors almost universally seem to make it hard to run stock AOSP (and do the Windows bloatware thing that Windows vendors were known for), so a "linux phone" lets people run what they want and remove what they do not 2. AOSP, while open source, is not developed in any way like a community open source project, so their ability to change anything, especially anything Google does not want to change, is limited and means constant rebasing 3. AOSP doesn't really solve the "run a modern/non-buggy kernel" issue on existing vendor hardware (as far as I know), so if you're going to spend time on getting the kernel to work, you probably want to have a userland that is amenable for getting the kernel working, so AOSP isn't helpful there, and by the time you've done all this, you can probably just run the rest of the standard setup with a distro and tooling you are already familiar with

I think the interesting thing would be if the modern kernel work from (3) could be used by an AOSP build and get the best of both worlds, or if by the time you do all this AOSP is too resource intensive to run on the device, and so running the alternative is the only option.