He has literally no combat feats. That's just not what his character was introduced for - thus it would be against the character to throw him into the games and then create a bunch of combos for him. It's the same reason why we don't have other playable characters that either didn't have much screen time or wasn't meant for combat.
I used to want him in a game because I agree, it would be cool to have the complete Akatsuki team playable - but let's be real. You would never touch Zetsu if they created fake combos for him. We have the Zetsu armor Obito, which is perfectly suitable to me as a substitute for the real Zetsu.