Because you can’t/don’t integrate major process changes at individual steps. You introduce entirely new or revised processes with many steps updated or new tech

Because you can’t/don’t integrate major process changes at individual steps. You introduce entirely new or revised processes with many steps updated or new technology introduce in tandem …so called nodes. When you move to suv, for example, you need finer etching and better cleans and many other things. Until you introduce Euclid if you don’t need the finer etch and your process “yields” well don’t fix it if it ain’t broke. An analogy would be…star drive screws are better overall than philips screws. But I probably wouldn’t switch my car assembly line over to star drive screws by doing it one assembly station at a time. I’d shut the line down for a weekend and change them all, or I might change them all as part of switching production over to next years model.