People, not just women, like to avoid confrontation. By not telling the person to go away, you avoid confrontation. It tends to be the path of least resistance.
I bet most guys would do about the same thing if put in their shoes. I mean, if girls were texting you regularly trying to escalate, would you bluntly and politely tell all the ones you didn't like, every single time, "Sorry but I'm not interested" ... or after a while of this would you kind of just not give a sh*t and simply stop responding? Confrontation avoided, check. Ego boosted, check.