Because if a girl walks up to a guy and says i wanna use for sex, he'd probably be all up for it.
Girls though probably wouldnt, so guys have to manipulate them to get what they want.
Saying that though, its happened to me before and 100% honest, if he'd just said to me 'look, i dont want a relationship, i just wanna ****', i wouldve been up for it. I lost any respect for him when he tried to manipulate me into a relationship.
You're right, people should be honest about their intentions