Cost is a big factor but it is not the only consideration. Even if cost was no object and you could just throw a lot of machines at the problem, designing distributed algorithms that will scale to thousands of machines is a different kind of problem than what you can get away with on several dozen machines especially when talking spatial access methods (which have their own unique nuances).

This is surmountable but few people know how to design the data structures and algorithms required to make non-trivial spatial data models scale to that level. There are significant gaps in the published literature.