Why is it so bad for friends to straight up ask each other if they want to be friends with benefits/hookup?

I (M21) always see post on here where people are talking about how they hooked up or are now in a friends with benefits relationship with one of their friends and one of them “just made a move“ or “one thing led to another” but they never discussed what type of dynamic it was gonna be like if they wanted to be in a relationship or they didn’t do that until after

I don’t understand why is it so bad if a friend ask a friend that question if they’re being respectful about it, also why is it even worse if a guy ask it to a friend of his that is a girl? I will see people in here then we’ll talk about how communication is what makes sure that it doesn’t ruin the friendship, but apparently this is too far and I don’t understand it.