The short answer is that Apple decided sometime back that were they to move to two buttons (we're talking late 80s here) that the advantage of the second (contextual menus) would become a crutch for 3rd party software developers. (this was the same reason the Mac didn't ship w/ a command line interface; ISVs might not have made GUI programs but just had all their programs remain text driven to run in "DOS" boxes) Given an easy assumption that users will eventually try the contextual menu, they would cease to bother to have interfaces that provided functionality in an intuitive fashion. This proved to be a reasonable concern (you do see context menu hell in Windows apps)... and yet for gaming more buttons is a necessity.
I just have two; a one button bluetooth mouse for most things, and a wired "gaming mouse". I find it a reasonable compromise as I don't like using wired mice most of the time, but for games few wireless mice can be relied upon.
-RS