It always puzzled me as to why we don't see much of that.
...but I guess if you're Zynga and you're serving games to 70 million users already, there's not much incentive for you to go chasing up character license holders to split the rewards with them.
It also begs another question - since the nature of these social games is you "create" your character, they seem to place much less emphasis on characterisation. Are we ever going to have this generation's "Mario" for example, or is character branding like that going to become extinct if the long term trend is social gaming?