I think the mindset has a lot to do with it. The type of person that trains TMAs is often passive and laid back, training more for fulfillment and basic self defense than an actual brawl, while the MMA/boxing mindset is all-out ground-and-pound. As a result, TMA-ists are generally afraid to train against a resisting opponent that's genuinely trying to brawl with them rather than someone on the street that tries to incapacitate them quickly with one or two things that they specifically train against, and if they do begin to train in a boxing/MMA gym and get their *** kicked, it's much more convenient and much, much, much less time consuming to switch to something that's been tried and proven than to step back and analyze what did and didn't work.
With regards to grappling specifically, TMA people tend to be pretty naive about it, usually with a "well, if he grabs my legs I'll just chop him in the back of the neck" idea when:
1 - With the forward momentum of that particular takedown, your hand is more likely to bounce or miss completely than actually strike his neck
2 - That's not the only way to get taken down.
With regards to grappling specifically, TMA people tend to be pretty naive about it, usually with a "well, if he grabs my legs I'll just chop him in the back of the neck" idea when:
1 - With the forward momentum of that particular takedown, your hand is more likely to bounce or miss completely than actually strike his neck
2 - That's not the only way to get taken down.