From a production point-of-view, they were probably quite an expensive bit of kit to have manufactured, and stored, so to an extent they have to use them, to get their moneys worth.
Yes, it would be nice if every episode had a new type of monster in it, but the Daleks are assembled already, so the most significant cost of using them is probably the guys who have to be inside them. If you've got some new random humanoid alien, you'll have to pay for the actors who are dressed up, and the special effects artists who have to do any prosthesis and make-up that they'll need.