I think like any problem there is not one simple answer, but rather a combination of several factors.
When you do not understand that martial arts and fighting are two different things, then there is a tendency to assume that anything that does not work when it comes to fighting/combat sports, does not work full stop. It seems of course obvious to a lot of us that if something is not designed to score points in a combat sport, then the fact that it does not work when you try to use it to score points in a competition is unsurprising.
Some however choose to take this as evidence to support their belief that it simply does not work anywhere at anytime in any context.
If your area of interest is competition combat sport, then it would make sense to concentrate on techniques which will bring you success in that field, hence you wouldn’t train in something which won’t (Kendo, Aikido, self protection). Again however, instead of being obvious to some people, take the lack of “Aikido fighters” in MMA as proof to them of its uselessness. Even experienced martial artist like Joe Rogan and Ronda Rousey don’t seem to be able to differentiate between martial arts and fighting/combat sports, and lump them altogether as one, as both have cited the lack of success of Aikido within the arena of combat sports as proof it doesn’t not work full stop.
Of course what we should be saying is it does not work in situation/context A (because it isn’t designed to) it does however work in situation/context B (where it is designed to work).
The opposite of course is also true. You cannot explain to a cage fighter that a triangle choke does not work for self protection. He has used it successfully in the cage, so he *knows* it works. It is only when he tries to use it outside the chip shop on a Friday night and gets his head stomped flat by the guys mates that he realises just because something works in situation/context A it does not follow that it automatically works in other situations.
Part of the reason is youth. When you are young you just assume that because you train for fighting/combat sports then everyone does too, and that anyone else who trains in other arts for others reason is wasting their time with “**** that doesn’t work”.
Part of the reason also is the way Aikido presents itself. Invariably Aikido is shown without the accompanying atemi required for the techniques to work in a live situation. People who have made no effort to understand Aikido take what they see as being “the whole picture” and say it doesn’t work, not understanding that there are other things happening of which they are ignorant because they are either not shown, or not immediately obvious to the viewer.
Mix these together and you have bunch of people who *Know* it’s **** and doesn’t work. One day they will get older, and wiser, and perhaps appreciate that other people train for other reasons, and that somethings work well in context A but are wholly inappropriate and not designed to work in context B. Problem is, they get replaced by a new generation who think like they used to
That is not to say of course that there isn’t bad Aikido out there. The third biggest waste of time I have spent on the mat was my first experience of Aikido at a multi style seminar. We spent an hour shaking hands and then falling over because we didn’t want to embarrass the instructor.
Fast forward a few years and I have experienced Aikido again on a number of different seminars with a different instructor whose Aikido does work, oh boy does it work!
Aikido isn’t alone in having bad elements though. You can find bad examples of any art, MMA included.