Because it makes the game more fun.
The only gambits i use are stuff like hp < 20% and shit, or self: protect, or self: berserk.
By limiting your gambits and making them based solely around how you want your character to develop, the game is 99999 x more fun. I can only imagine this is especially true in the International version.
So basically, what I'm saying is, if you've got a list of like 12 gambits, and you're complaining that the game "plays itself", whose fault is that? You need at most, 4 or 5 gambits at any one time, and you can make it so that each character has their own gambit palette or routine. If there's one flaw with the gambit system, it's that they're not all unlocked from the get go and you have to buy or unlock some of them, which is batshit distinguished mentally-challenged.