Yes, I can see that happening! Did you use a heat lamp in the brooder? I put a little roost in the brooder from day 1 and my chicks were on it all the time. But I was using mama heating pad so at night they went to sleep under there when they were still small. In a week or two they slept on top because (I assume) it was higher off the ground than the perch, and probably a little warmer. I raised the pad up higher so there was plenty of room underneath but they went on top. Same thing happened with my first batch of chicks too! So when they went to the coop it probably seemed pretty normal to get up on the roost bars.
I'm just wondering if the OP's chicks ever had the opportunity to try a roost when they were babies. I wonder if chicks grow up without one, will it seem normal to roost up high?
I've just heard other stories where people had the same problem trying to get their young birds up off the floor to roost. I does seem pretty strange that mature birds would be sleeping in a pile on the ground.