Because the torque converter releases when your foot is removed from the throttle there is no fluid coupling to pass the energy from the rear wheels to the engine... .
Several vendors offer a kit... . that interfaces with the ECM on the engine and transmission to hold the fluid pressure in the torque converter, thus locking it up... . and allowing the power to be transfered through the torque converter so the engine and exhaust brake can absorb it... . We have them... .
There is no financial reason to offer a upgrade based on the cost of design and the overhead that Dodge has to upgrade trucks in the field... .
However they seem to jump on the wagon to take on the after market people when new model time rolls around... . we now see, cruise, AC, power locks, windows and now exhaust brakes as standard equipment. .
We've installed a good share of exhaust brakes on trucks with auto's... . I personally don't think that the cost is justified unless the transmission kit is installed... . once that's done..... the brake on the auto transmission functions at about 95% of the level of the manual transmission...
I also feel that the 06 and above is a better match than the manual because of the software... . we have waited until 08 and the Aisan Transmission in the 5500 before we jumped... . as we didn't consider the other automatics worthy of the work and only time will tell if the Aisan is up to the task... .
If you have an interest in a kit for your 05 with an exhaust brake we have them... send me a note...