For the same reason they don't have a list of their fares published on the ticket counter... because there are dozens of combinations of possibilities because there are multiple levels of awards depending on how many miles you are willing to spend. There is no ONE award level like there is on other carriers and award seats are revenue managed just like regular revenue seats - the more you are willing to pay, the more likely you will get the seat you want.
Why don't AA and UA have fare displays behind the ticket counter?... that was the case at one time.
in the mean time, every DL passenger enjoys more amenities than those of its competitors.
Don't hold your breath waiting for the FA to ask if you want peanuts, pretzels, or a cookie on AA or UA or be surprised if your bag is 2X as likely to NOT show up on the carousel if flying AA, UA, or WN. and bags or no bags, your plane is less likely to arrive on-time - if it even operates - if you are flying AA,UA, or WN.