The supposed "sheep" that want to get on the plane first are people that want to get that precious overhead bin space to avoid checking a carry-on bag at the gate. Boarding last means there's no more bin space and the gate agent will put the bag in the belly of the plane. This adds extra hassles of waiting an extra 30+ minutes at the arrival terminal to wait for the bag on the conveyor belt and/or the bag getting lost.

Yes, it can look "irrational" to hurry up and get in line because as some like to say, "No point in fighting to get on the plane first since we're all leaving on the same plane at the same time!" ... The issue isn't the departure time -- it's the limited bin space.

EDIT add reply to : >bag put in the belly lf the place, and my bag was never lost.

There are more complications because at some airports with widely separated terminals, going outside of the security zone to pick up a bag at the conveyor belt also means using slower buses instead of the tram to go to another terminal to get a car. E.g. at Dallas airport, the faster railway trams are only available inside the secured area. So not getting that bag in the overhead bin has domino effect of waiting for buses (another +30 minutes) which can add up to 1 extra hour of waiting at the arrival destination. Getting in line early for boarding is a small price to pay to avoid all of that.