WarlockLord said:
The usual answer to this question is 'as many as the market will bear'.
Isn't one big book enough[?]
Can you come up with every cool monster you might need for the first book? I highly doubt it. How big should the one book be? 1000 pages? 500?
Doesn't WoTC print enough?
See, there's a thing called 'cash flow'. Like it or not, WoTC is a business and for a business to survive (especially a business that has to answer to stockholders), new sources of revenue must be found and generated. New books are created for this purpose as well as to provide new cool stuff for your campaigns. Also, there is a PR angle to consider. Is it more damaging to have people saying 'Wow, WoTC puts out a lot of stuff' or 'Hmm, WoTC hasn't put out a book in three months. I guess they're giving up on D&D'.
Aren't there rules in the MM for creating your own monsters?
Certainly there are. Because of time constraints and such, not everyone has time to come up with 200+ monsters on their own, even assuming they have the imagination, energy and talent that a half-dozen people who do this for a living possess. You might have that, but I sure don't.
Because they may not be the right thing for the job. Maybe another monster has a cool special ability I want to use to bedevil my players, or has an interesting backstory that I can springboard off of. Maybe I want more planar monsters or devils or undersea creatures; if it's not in the MM and I don't have the time or energy to go off and create a batch of cool monsters, I guess I don't run a planar or underwater campaign for the group.
Do most people even buy these?
Obviously enough must for them to do more than one.