Everyone moves in their own, individual, way - A combination of personality, experience, injury, and training.
We all have a way we walk, run, dance, and if you train a martial art, it's quite easy to see which one, sometimes even who your teacher is, by the way you move when you do something, even if it is in a different art.
Put a sword in someone's hand and all their previous movement patterns will come out. Some will be useful - We already know how to dodge, fade, and run away from playing games when we were kids (though of course freezing when faced with a sword may prevent this, if one are not used to playing with one).
All the rest of the stuff that has been trained in from other martial arts may or may not be as useful, especially if the conscious, trained, stuff overlays the subconscious, 'kid', stuff.
Add to this that you then have to train IN a bunch of overtly counter intuitive stuff ... and it becomes an interesting process.
It seems that the only true motivator to change ingrained habits is from repeated exposure to moments when they fail. If these moments do not occur, these tendencies seem very hard to work with, as the person doing them does not seem to have conscious control of them ..... and with no incentive to change, they repeat.
Even with failure on the table, there often seems to be huge resistance to change these familiar patterns to something smarter, and even those that intellectually might want to change ... find it hard to actually change.
I guess our brains hold on to movement patterns at a pretty deep level, and as they are connected to our self image, our identity in how we present ourselves to the outside world, there has to be a mental shift, and perhaps emotional too, before the physical can also free itself to be new things.
Addendum: After I wrote this piece, I read a study that may point to another reason why:
Here is a quote:
"The problem," says Bjork, "is that if people confuse the current sense
of ease with learning, they'll tend to prefer training conditions where
things are kept constant and predictable--conditions that act as
crutches to prop up performance without fostering learning."
Once again, thanks to Steve Morris for the link, and his eternal quest to find the science behind movement and learning.