Actually I haven't seen many small aircraft and no empty flights in the
US for the past several years.

I don't know where you've been, but it's been quite a while since I've
been on a domestic flight on anything bigger than a 737 or A320. Last
week I flew ITH-PHL-ATL and back on US, and all four legs were on RJs.
Rather full RJs, but I'd think that PHL-ATL could support bigger
planes.

My working assumption is that the crew costs on RJs are so much lower
that they're willing to lose the revenue they might have gotten from
selling more seats on a bigger plane.