Project Treble

One difference is that Apple gets something like $10B/year income from the App Store (vs about $100B/year income from iPhones, but the App Store income is presumably all profit), so they make a non-trivial amount of money even when customers aren't buying new hardware. Android device vendors don't get that, since the app income goes to Google.

Apple also sells a much larger number of units of each model than most Android vendors, so they can more easily afford the per-model cost of developing updates. And their scale lets them do a lot more of the hardware and low-level software in-house, rather than relying on third-party vendors, so updates don't depend on negotiations between multiple companies with not-necessarily-aligned goals.