I would challenge the use of "so many", though maybe your experiences have been different to mine. However while the OP asked about USA flights, I note several references above to SIN - that is kind of different since these flight departs LHR around 20:00-21:00 hrs, with the 787 being on the later service. That is 13 hours long, arriving around 17:00 hrs into SIN, so it's a bit of hybrid with a substantial period - 7 hours or so - of darkness. So for much of the flight it doesn't matter and then there's a debate to be had as to whether / when to undim, which at most affects the last 5 hours or so.