This is the case at IAH in the United terminals in C and E. The airport website says that both are open all day from 5 am to 9 pm, but in reality it's more like 5 to 8 in the morning and 5 to 8 in the evening.
They say it's about "not having to pay screeners to stand around when no one is there" but that's mostly nonsense. Like you said, the regular security line is long but the precheck line is closed, so the explanation makes no sense.
It's very frustrating because sometimes I want to take a flight later at night and if I there is no Precheck.
Frankly, even if I do get do keep my shoes on, having to pull out my laptop and my toiletries is still a pain in the rear, especially since it affects the way I pack.