It's not a matter of "doesn't need to." Most people, if they simply do something enough, and work with people more competant than themselves, improve just because of repetition and some of their colleagues' expertise "rubbing off" on them.
Usually the only way you don't get better in that situation is out of contempt for the job. I really don't think Cena has contempt for wrestling or doesn't care...I think he must just be incredibly unathletic and dense about wrestling--IE he has ZERO "natural talent." And it's just amazing that a guy like that can find himself at the top of his field for years in a field that's supposedly so competitive.
I dunno man, I think he's passionate about the business, but you're right about no natural talent. I think he's at the top simply like I said above, he sells Tshirts, he is big in Make a Wish, and he is the key figure in WWE's new target audience, the 5-10 year olds. It's sad, but it's where they've gone. Ability to sell tshirts and cater to kids > Talent in Vince's eyes.