The girls may not have had a chance to scream, or may have been too scared to (as counterintuitive as that is). If they did scream, it’s entirely possible that no one was around to hear them. I don’t take this as any indication that this guy is experienced, in fact, I take his actions as being a sign of the opposite. He could have been spotted walking in. He could have been spotted on the trail. He could have been spotted on the way out. Someone could have seen the attack. Broad daylight attacks are inherently risky, and we do know that people likely saw him at various points. Libby was able to capture him on her phone, and he neglected to remove it. He’s incredibly lucky that he wasn’t caught right away.