They got boring and repetetive really fast now, before Tales were great because when they came out they were pretty rare. RPGs with a very action take on it.
However they've used that exact same formula in all their games with the usual change in how you get SP or something. Tales of Abyss was the only game that really changed up the system with the arte skills. THAT was cool but Vesp, Xil, Graces barely did anything. Graces was a bit more fast pace but the story was very boring. Well I lied Xil did add the new duel mechanic but it still plays the same.
I hate to bring up Final Fantasy in the mix, but even though I don't like XIII or XII that much they did always bring up new mechanics.
VII had the Materia system for equipping magic instead of just obtaining it. VIII Had the Draw system, and more ATBish mechanic, and a card game system IX was meant to be a throwback to classic Final Fantasy a new card game system X had playable summons, blitzball, more random mini-games X-2 had real ATB X-2 was really fun (Though realy bad plot) and brought back class changing. XII is a really open world, more action oriented with a whole new system implemented.(Too bad the game itself was boring) and XIII same as XII, a new overhaul to the systme which was good but the hallways and story and characters were just UGH.
Kingdom Hearts also always switched up the mechanics in every one of their games strongly.
Quite honestly I think it's just a Namco Bandai thing, a lot of their games rarely face changes the only one I can think of that did was Ace Combat Assault Horizon everything else just retains to the same old formula over and over. I think they're just afraid of change cause whenever Sega, Capcom, Square-Enix ect do it they get hassled for it.