Personally, I don't find AI all that challenging to make in my games, just time consuming. I'm willing to bet bots could be put into SRB2 but it would likely take the addition of game elements that really aren't thought through and would take an immense amount of time to add.
For example, if a bot wants to get to a specific part of a level, it has to know how to get there. Each level would have to have basic navigation paths built in for the bots to follow. That alone is probably way too much effort for the game in its current state.
The difficult part would be testing not only the difficulty of the bots but their predicatability. An AI that can be predicted is no fun to play against, likewise an AI that is super-human yet still playing by the rules is also not fun. (Good example, Mechwarrior 4, set the bots to skill 8 of 9 and they still miss with a certain frequency, set them to 9 and they NEVER miss.)
Not to metnion the sheer amount of time it would take to program the bots behaviour. AI programming is more or less "Watch what the AI does and when it does something stupid program it not to do it."
Instead of bots in SRB2 we should have spectators :)
--- Gemini