Because people don't want just another android phone. They want their desktop on a phone. They want to use desktop GUI toolkits, desktop package managers and all the rest. Android also contains a mountain of anti features that are hard to remove. Its also missing a lot of critical features without the GApps so maintaining a fork of android is almost as much work as making GTK work on a phone.

PureOS was already shipped on their laptops. Debian (what it's based on) already supports ARM as well as many other architectures. Lots of software works with basically no needed change because of their decision. The only changes really happening are to make things work better on a small touchscreen, but no one's stopping you from running the desktop version of mpv and plugging in a keyboard. (or using any terminal programs, which all seem to work great)

I even saw recently someone was running openmw (morrowind) on the Librem 5. Stuff doesn't have to be ported to Android this way. There are so many more exciting possibilities.

They've got support similar to a Raspberry Pi or other SBC out of the gate in addition to being a phone. It's like a real computer.

It strikes me as simple impatience to want Android on one of these phones. By taking the time to make a better base to build on, we can be free of the current ecosystem and its problems, it may just take a bit longer, or maybe not every average Joe will want to be a part of it, but the end result should be much better.

If you really do want an aosp base, though, there's glodroid[0], which supports the PinePhone. I haven't seen any reviews or tried it myself, though.

note: I do not own a Librem 5 or any other Purism product, nor do I plan to get one in the near future, but I have a PinePhone and I'm very on-board with the direction things are going.

[0] https://github.com/GloDroid/glodroid_manifest