It's the money and the fame. Tiki recently went through a costly divorce and his new trophy wife has expensive taste. To add insult to injury his once promising TV career has been put on the shelf leaving him with few options to make money. Once you've achieved a certain amount of status it's hard to go back to being a normal guy. It's one of the hardest transitions for athletes to make. It seems simple enough to acclimate ones self back into mainstream society after your playing days are over, but few things (if any) can replace that void once the game is over. Isn't that why Bret Favre played into his 40's?
Comebacks have been attempted over the years and with the exception of Michael Vick (much younger than Tiki) and Mike Williams (much much younger than Tiki) none of them were successful. Four years is an eternity to step away from the game. The players get bigger, stronger, faster, and more determined. Antonio Pierce has publicly called out his former team mate for throwing Eli Manning under the bus, criticizing the coach, and exercising poor leadership qualities in the locker room. The New York Giants organization has gone on record as saying they want nothing to do with him. These are not exactly ringing endorsements for a triumphant comeback.
Desperation and greed are not good reasons to attempt a comeback. I see this experiment lasting half a training camp. At that point Tiki will realize his body can't stand up to the physical demands of football anymore and his heart and mind aren't in the right place to be successful. Hopefully for Tiki's sake he can stick around long enough to tape a few episodes of a reality show based on the experience. Isn't that how they do it nowadays?