Why are these Aikido fighters not mopping up and earning millions in MMA?
The philosophy is one of defence not offence. The same with Tai Chi and Kung Fu.
MMA/UFC use styles of fighting that are predominantly designed for competition.
Technically they use mixed fighting styles taken from Martial Arts and not the Martial Art itself.
It doesn't mean that Tai Chi/Kung Fu/Aikido are ineffective, it simply means it is not MMA.