This one seems like a big problem to me. Maybe the handset manufacturer doesn’t care because they already made their profit, but this pushes the support burden onto app developers (including Google itself) who have to maintain support for these old Android devices that the manufacturers don’t care about.
This seems like a real issue to me, is there something I’m missing?
Back on topic though, I think it's just easier from an engineering perspective to maintain a stable ABI than it is to maintain a set of blessed kernel versions.
An ABI is something that can be reasonably well-defined and has a clear scope, whereas if you just say that you support kernel versions X, Y and Z, then who knows what weird undocumented behavior you'll need to maintain.
https://lwn.net/Articles/771974/
Also some vendors such as Sony have standardized kerbel versions across many devices & publish kernel major version updates for a range of devices (in Sonys case called the open device program).