For the reasons I just said - they either place themselves in direct danger in order to protect those under their charge, or their general conduct as a leader makes them easier targets.
If you work on the basis that everyone in these movies is at risk from the aliens (which they are) then you have to look at whether their individual conduct makes them more or less likely to be taken. Newt, for example, did things that greatly reduced the risk to herself, while each of these leaders did things which greatly increased it.
I'm just saying that I don't think the aliens sit around strategising, thinking "Yeah, lets go and find the leader so that the rest will be demoralised". That's what usually happens, but it's because of modified behaviour on the part of the human individuals, not the aliens.