Training 3 times a week to stay in shape is A LOT different than training for a fight. I am sure Rampage wouldn't hitting the gym now and again to land movie roles.
Staying in shape and landing the "Mr. T" roles are two different things entirely. Let's look at at a recent action star like Chris Hemsworth (Thor) - You think that guy was hitting the gym 3 times a week for that role?
Action roles are about all Rampage has to look forward to because his acting sucks. He might land a comedy role (at which point he can get fat to his heart's content) but for the action movies he's mentioning he's gonna need to stay in the gym to get shredded.