What incentives do professors have to serve on dissertation committees? It seems that there is no requirement for professors to serve on even a single dissertation committee. (This is so at least at my US university and I believe is common practice worldwide - please correct me if I'm wrong.)

So why would any of them do it, other than out of the pure goodness of their heart? It seems like purely volunteer work.

(Of course, one can imagine professors being eager to serve on the committee of a John Nash or a Ludwig Wittgenstein, if only so that their names appear as a footnote somewhere in the history of science, but most of us are not Nash or Wittgenstein.)

[Personal anecdote: I am asking this question because I've been having some trouble filling my committee. Many professors simply respond that they have no time, are too busy, etc. I am left feeling like a pathetic beggar grovelling for favors, even though I may be paying hefty tuition fees, which presumably helps pay at least a little for their salaries.]

Addendum: I simply wanted to give a little context for my query, but it was perhaps a mistake for me to add the above personal anecdote.

I'd prefer answers to stick simply to the question itself, which to repeat, is quite simply this: "What incentives do professors have to serve on dissertation committees?" 

Perhaps we can leave to avenues other than StackExchange the opinion-based debates about whether the current situation is ideal or whether I personally have a moral defect and should change my "client" mentality.
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.