Is a part of the problem that the chips they are depending on are so trivial and small that you would end up losing more when cutting it up on a smaller process? I guess there is a limit to how many chips you could put on a single wafer even if the chip was only a single gate?
Maybe the old nodes are efficient enough for the chips they need. Then again it would make sense to update designs frequently enough to stay at nodes that have plenty of capacity, perhaps every 5 or 10 years or so?
Capital costs of a fab are the single highest cost of producing chips. So long as the fab is running it's cheap to keep running so little reason to spend money to put cheap things on other processes (which is an expensive design cost) when you can later switch to a more complex chip (which should allow you to reduce supporting components too) which takes advantage of the new process size.
Maybe the old nodes are efficient enough for the chips they need. Then again it would make sense to update designs frequently enough to stay at nodes that have plenty of capacity, perhaps every 5 or 10 years or so?