How about "because if we sent two or three people to the hospital every class, we soon wouldn't have a dojo"? I've sparred in taekwondo and karate, and I confess I can't imagine anything like sparring in aikido that wouldn't result in a lot of non-trivial injuries.
Or not. The Shodokan stylists seems to have found a way to do sparring in a safe format.
I dont think sparring is necessary, but honest jiyu waza where uke is free to escape or reverse techniques is.