It's because they try to make them look pretty and it in turn makes the game suffer. Sure they look pretty good but they feel sluggish. Not to mention THQ has owned the license for years, no real competition for WWE outside of TNA and the last TNA game tanked.
So you have the same game year after year after year with only minimal updates. I'm hopeful 2k can get it right, atleast 2k does tend to change up their sports games once in a while.