We were unable to find the exact meaning of "you can't milk a cow you don't have." What We found is:
”Why buy a cow when you can get milk for free?”
This idiom means why pay for something that you can get for free otherwise.
Sometimes it is used to refer to men who don't want to get married, when they can get all the benefits of marriage without getting married.
Check some idioms similar to 'Why buy the cow when you can get the milk for free' at Usingenglish.com.