@marcellinusme
There can be plenty of reasons: There's the hacker mindset of "Why not?"; Building off of a mainline distribution gives one access to everything in that distribution; Likewise, contributing to making a phone possible does the *opposite* (e.g. I can make my laptop a "phone"!); It is impossible to have a fully functional FOSS userspace phone with AOSP; Google is working towards making Android less open.; Don't build your business off of someone else's product.
@awai @kop316 @marcellinusme Add to that the fact that many Android apps don't even bother to start when they hit a non-vendor Android claiming it has been "rooted". Same story when you want to run without google play services.