Maybe Pilot Hunter should be 8v8, but all of the other modes would be severely impacted by increasing player counts. Killing AI units is useful for points, but more importantly for reducing titan wait-time. They're used as a trade-off: Do I focus on the mission of the game-mode, or do I stay passive for a couple of seconds to reduce my titan time by 30. Does my team need me to speed up my titan (since theirs are ready), or should I focus on capturing/holding this point?
There is a clear game mechanic for having the AI in the game that would be completely broken if they were all replaced by (fractionally) players.