While I agree it seems like there are express jets running all day on every domestic flight, the truth is that there are roughly:
55 x Airbus A319
97 x Airbus A320
5 OR 6 x 737-500
36 x 737-700
130 x 737-800
43 x 737-900
134 x 757-200
21 x 757-300
5 x 767-200
35 x 767-300
16 x 767-400
55 x 777-200
23 x 747-400
and other than a handful of grounded 787 aircraft, these planes are flying pretty much round the clock to various destinations. It can't all be express jets