One reason historically was that the knuckle on the price curve shifted between the time the machine was released and when you upgraded. When a new generation of RAM came on the market, prices often fell rapidly as the process matured and competition set in.

This also played into sale or discount strategies: if the same model becomes cheaper over time but list pricing doesn’t adjust, you’re making a good bit more from everyone who isn’t price sensitive even if you allow resellers to discount it periodically or offer better pricing to business orders (“free RAM upgrade”).