Payload for 2016 Ram 2500?? What am I missing??

You are not missing anything, the payload is mostly limited by the Class 2 GVWR max of 10,000#!
You really have two options;

#1. Ignore the GVWR, and go by the rear GAWR, which gives you about an additional 1,000#.

#2. Far better if buying new, just get a 3500 SRW! You get about 1,500# additional pounds of payload, and you will be within all your ratings.