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

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.