Whatever is said on the topic, dancing is a spectacle. Force someone untrained to make a spectacle of themselves, and prepare for embarrassment and anger, and potential resentment. We -generally- don't like to get things wrong, especially in front of others, and so accordingly minimize the chances of it happening. Ergo, no dancing.
The reason why, is probably because it is a formless expression of feelings pushed into a slew of incoherent movement. I am not bitter about dancing, I like dancing, foxtrot, tango, salsa, whatever. It is simply that there isn't any social protocol that defines what supposed modern "dancing" is supposed to be, and that freaks the hell out of a lot (possibly most) happily conventional guys. Sure we are happy being silly, alone with our partners and friends, but put it in a public place and all that intimate safety is removed.
#1 because we don't look nearly as good #2 Don't jump on your bed with young lady! #3 first impression "sexy imagery" second impression "you do realize you can hurt yourself that way, don't you?" #4 you should be paying attention
These are generalizations so of course I realize they don't apply to everyone, but from what I've observed, generally guys are expected to maintain a more cool or serious appearance, so dancing silly is kinda off the table if you care about those expectations. Also I'm pretty sure a lot of guys don't actually learn how to dance well, so dancing silly is their ONLY option.
I'd be nervous going to a dance simply because I don't know how to dance. I can (and have in the past) improvised, but my improvised dances are always silly and I'm not sure that's something I'd want to bust out in a public venue. But on YouTube? SURE! xP