It is really simple. There is no battle or physical combat during aikido class. The attacker works together with defender. He is doing prearranged attack, and his behavior right after the end of the attack is also a prearranged form. You can't call it a battle, rather some kind of theatre play.
Also, the intent of attacker is not to harm defender. Rather, attacker allows defender to learn.

Well, there is no cooperation in MMA and the intent of attacker is to harm defender in the limit of the appropriate rules.

That situation is a result of Founder realization that martial techniques can be used to change/improve ourselves (i.e. misogi) and not only for fighting.