As a point of comparison, Microsoft ships its OS across a variety of manufacturers and largely keeps it maintained across them (give or take some exceptions like enterprise environments & the like).
Even unlocked Android phones purchased independently of carriers have inconsistent lengths of support, so it doesn’t seem to be entirely a result of carriers, so…What happened here?
There are other custom ROMs as well which get frequent updates monthly. Like my Lisa got official LineageOS support after 2y of its release but there were other custom ROMs during that time and even now.
Corrected. The custom rom support sometimes stops with a major release change, but there are unofficial builds as well.