Fred Trump threatened to disown Donny's brother for even wanting to join, as I understand it.
Donny personally would never be able to serve, because it involves sacrifice.
I'm sure he hated taking orders in his military academy life, but loved giving them.
And don't forget, we wound up with this POS POTUS because his pimp grandfather dodged his military service in Bavaria, and got kicked out of his home country; so maybe cowardice is hereditary.