Because you will not get as good of a deal compared to Costco. You get a Costco cash card worth 8-9% of your cruise cost. you can pay on their website in full or a little at a time.
If you need to change your cabin (not done very often) then you just call costco travel, they call
DCL with you on the phone and it gets done.
Everyone has the same price as DCL, there are no discounts. But get the best deal like the OBC or the Cash Card..... whatever is higher.