However personal interest (i.e. greed) is a big factor that motivates them to devote their lives not to something that's simply very interesting to them personally, but rather something that just happens to be practically useful for large numbers of people very unlike themselves.

Smart people who have the ability and resources to change things don't really share the same problems as the target audience of mass market tech (I mean, hackernews is full of product reports illustrating this) - greed motivates them to solve problems of their "outgroup", filling the needs of all the people they don't really care about much.