What incentives do professors have to serve on dissertation committees?
The professors serving on one committee will also have their own students, who they will want to see graduate (even if you assume pure selfishness they want their recrords to look good). Those students will need to be examined as well so in a sense it's mutual assistance, but time-deferred.  Also good relationships between academics are how a lot gets done: this is a fairly easy way for an academic to build relationships with other (perhaps more senior) professors.  So while generosity is important in this sort of thing, it's not quite that simple.

As an aside, you might like to ponder the effect on such well-established networks and systems of treating academics and their research groups as business units in a corporate entity.