Thanks for that info. They didn’t mention that part to me.
The reason is they have to ask the specific security questions for the US, and that has to be done by a different security group, which is only there 4 hours before departure of a US bound flight. I have no problem checking in for LAX or SFO at 10:30pm though, but I always have an airside hotel reservation.
In the before times when JFK was 4 nonstops daily there was a late afternoon departure around 4:30pm which would theoretically make it possible to spend a lot more time airside, but no airport is that interesting and the F lounge is only so good. I never took advantage of that.