I mean I played 2v5 customs and got no IP, so I'm not sure about that. That said, I'm not 100% certain that is the reason why they don't implement Hard bots, it was just my guess.
I certainly could imagine them considering Hard bots when they add in the sandbox mode. I mean I agree with you that there should be Intro, Beginner, Intermediate, and Hard bots. Each should be geared towards helping new players to learn the game and should eventually make players able to play PVP without feeling terrible about themselves. I mean, they don't even INTRODUCE jungling in bot games. Not once are new players exposed to vision control, jungling, counter jungling, dragon/baron contests, etc... All of this is thrust upon them when they enter normals, yet people flame when someone wants to go duo top.