Crossdressing (in some regions) is considered a threat to masculinity.
Asking why is like asking why is playing sports masculine?
Where I live sports are a big deal. Too big a deal. It is nothing for some parents to force their son to play sports. In some families, the girl is forbidden to play sports. Because sports is a guy thing.
It's almost like they're trying to prove that they are men.
It is a mindset. One that cannot be challenged. I know too many of these men who are obsessed with being a man to enjoy life. They are too concerned with what society is telling them to do.
Many of these men are dying too because of the stress they feel to maintain their masculine identity.