Ask HN: Why do smartphones and SoCs require a custom build per model unlike PCs?

Take Linux distros for x86: there's a single .iso for all devices Linux supports at all, be that laptops, desktops, servers, etc. And my 10y.o. PC is still getting updates smoothly.

But then if we look at say Armbian, that seems to offer a custom image for every board supported, and those images are sometimes outdated for that specific model.

Same for Android, there isn't just a single Lineageos image, every device needs its own, and all devices are always outdated because vendors forget about them in 2 years at most. Question is, why is it the vendor (or custom LoS maintainer) that determines when and if my Android phone will be updated, but it's not up to say Asus if my laptop is going to get an update?

With Android I'd almost think it's done intentionally as part of planned obsolescence, but then why is it so with SoCs too and not so with x86?