The first answer that struck me was: Because so many good novels get rejected by readers.
Agents are readers too. Readers first and foremost, hopefully - usually people who work in the publishing industry start out as passionate readers. Quite often we're rejecting good novels because they're just not novels that we want to read. It doesn't mean they're not good. It means they're not good for us. The person who used this search term has no doubt gone to a bookshop more than once and ignored the hundreds - thousands - of very good novels because he or she only wanted one good novel. Only had time to read one good novel right then.
I reject good novels. I have to: I simply can't take on all the good novel manuscripts I read because (a) I can't physically manage that many authors and (b) I can't place that many good novels with Australian publishers, who are also limited in how many good novels they can publish.
And that's as much as I've come up with on this particular matter. Now I have to get back to cramming things in to make up for the amount of time the Sydney Writers' Festival will consume next week ...