That's easy. COST. It almost always comes down to cost. EDM is a painfully slow and expensive process and adds about $50 a set from a quick search on Jeg's. The factories couldn't supply the number or lifters required if there was an EDM process involved without investing cubic millions of dollars in production equipment. The standard design works fine for every day use so there is no reason for the factory to use them. Engines usually last past the warranty period so they are happy. Doesn't mean that there isn't a better or more effective solution for a high performance application.