Well, they could. But that's not a fair comparison as you can't roll up to London City five minutes before take off, and the Scottish airports are usually half an hour or so from the city centre. In any case the point of the sleeper is that you are multitasking - travelling and sleeping at the same time. If they get you to Edinburgh in an hour, or Inverness in 90 minutes, they will have to find you somewhere to spend the rest of the night.
My recent experiences: in order to get to Fort William for 10am in the absence of the Sleeper, instead of leaving the house at 7:30pm I had to get a taxi to the airport at 5:15 am the previous morning catch a flight to Glasgow in time to get a train to Fort William, and then check into a hotel for the night. Likewise returning from Inverness - instead of arriving in Euston in the early morning, I was still in an Inverness hotel, and didn't get to Gatwick until lunchtime.
To rub salt into the would, instead of being socially-distanced in a sleeper cabin, someone on the plane or in the hotel turned out to be Covid-positive, so a few days later we got a track and trace instruction to self-isolate.