Because they want something more physical than a musical instrument, for fitness or other reasons? Your argument seems to be based on a premise that anything martial-arty must be for self-defense, then loops back to reinforce its own premise. If we insert "gymnastics" in place of "Aikido", this argument never comes up. And there are schools (and, I think, an entire association) that market Aikido as being about peace and flow. If they aren't marketing it for fighting, then I'd assume most of the folks training there chose it for some reason other than that - perhaps even the reasons mentioned in the marketing.And then you have to think about the fact that if the same things can be achieved though other activities, like learning to play a musical instrument, then why would a person who is not interested in learning self-defense take up aikido?