Because when they break up she has nothing and he has her money in the form of equity. Splitting consumption bills is obviously good, but splitting a mortgage where one party gets it all is far less cut and dry.
If that's all up front and she agrees then whatever but the scenario in the meme is pretty scummy
Replies filled with people that will hopefully never live with their girlfriends because they seem very satisfied with the idea of lording over a romantic partner.