Considering that 'to dress' is a verb, why is it preferable to say "Go get dressed" as opposed to the simple "Go dress"?
Dress is one of those words where if there is no object expressed, an reflexive object like myself, yourself, etc. can be understood, like wash, bathe, clean, etc.
So you can tell someone to go dress.
However this usually means to dress for a specific reason, such as a ceremony, function, costume party, etc.
Go get dressed is idiomatic for putting on your normal clothes of the day.