If every
Munchkin genre had all the same goodies as every other
Munchkin genre, it wouldn't make a lot of sense to buy one over another. Giving everything its own little uniqueness adds to the variety.
In other words, the specific answer to your question is, "Because we didn't want to."
(Not to be a complete pimp, but there's a new Room in the
Exclusive Warehouse 23 Munchkin Booster.)