First, I worked retail for many, many years, including for (the awful) Walmart. I also worked at a restaurant for half a year. Nobody is at work at any of those places for more than about an hour before and after closing. I don't know where this 2-6 hours estimate comes from... but businesses can't afford to pay people to be there for 6 hours (3/4 of a typical 8 hour shift*) when they aren't doing any actual waiting on customers.
*And yes, it's a max of 8 hours typically, since most retail/restaurant employees would earn OT over 8 hours, and the Corporate Overlords hate that more than anything.
But more to the point, even if what you say is true about needing to be there at odd hours in the middle of the night, Metrorail doesn't have service at 3am-5am on ANY day of the week... with most of the week having no service 12am-5am (and 2 nights not having it 3am-7am).
So kk, I don't really understand this portion of your complaint... it has NOTHING to do with when trackwork is scheduled. Your complaint seems to be more of "why doesn't Metro have 24/7 service at better than 20 min headways to better serve low income people?" which is a completely different question from "when can Metro schedule trackwork to minimize impacts on customers and maximize the amount of actual repair they can do?"