Gronkowski has spent nine years getting in car crashes on Sundays. Going to a sport as physically demanding as wrestling may not be the best idea.
While the WWE may be scripted, the way these performers throw their bodies at each other is still painful. It's not exactly another human trying to take your head off, but it's still dangerous enough that Gronkowski should rethink putting his body through that kind of struggle. He has the chance to party for the rest of his life without having to risk injury to his brain or his body.
Plus his presence takes away a chance for an up-and-comer who has worked his entire life for an opportunity to perform in front of a WWE crowd. That just isn't fair to the other wrestlers.