At least when I still used Android, some SoC manufacturers made unmaintainable hacky patches for one specific kernel tree
(and sometimes the manufacturer would then add more customizations to make it even worse) and you'd be stuck on that kernel forever.
It kind of is though because Android allows for phone brands to tweak the stock OS and stop updating even if the OS itself is getting updates.
If Android forced all manufacturers to use the stock OS with updates, the phone could still get updates, no?