Why are vanilla AOSP Android ROMs not popular?

I'm a bit concerned with data collection by FANG companies, and thus still using a Nokia N9 and a Jolla Phone that run Meego and Sailfish, respectively. They are beautiful but dead platforms, so increasingly difficult to employ as daily drivers.

Android has matured a lot and is a quite nice platform these days. Plus, there's a wealth of good quality open software in F-Droid to replace GApps.

Till Linux touch-oriented userlands mature a bit with hardware like Librem, if that option ever becomes a reality, a de-Googled Android ROM is probably the only practical choice if you want an open platform without Google.

While I like LineageOS, departure from upstream and lack of reproducibility building ROMs makes me a bit nervous. Since vanilla Android (AOSP) plus F-Droid is pretty nice to use, why is it not popular?

That's even the option that Tor Project recommends: https://blog.torproject.org/mission-improbable-hardening-android-security-and-privacy

CopperheadOS was amongst the only ROMs that followed AOSP pretty closely, plus additional mitigations. Since COS recently imploded, there's not much else to choose from. But even self-compiling said ROMs is not too difficult for manufacturers that release tidy device trees and good documents, such as Google or Sony:

https://developer.sony.com/develop/open-devices/guides/aosp-build-instructions/build-aosp-android-p-9-0-0#tutorial-step-2