I can think of lots of reasons. My double bed is in the corner of a small room so I only have access to one side of it. Lifting the mattress to put on a fitted sheet is very awkward and pretty strenuous. This also means that there isn't much room for laying out both a duvet and duvet cover. Typically I lay them both on the bed at the same time because I don't want either to touch the floor which makes inserting the duvet into the cover even more awkward. Next is the fact that a double sized duvet is almost but not quite square. It's quite easy to grab a corner of the duvet and match it with the wrong corner of the cover. Lastly, I only do this about once a week not 10 times a day so I haven't had a need to find a better method so far.
Ours is against three walls - you can only access the bed from the foot. It’s also four feet up in the air as we put a closet under the bed.
Turns out you can put on a fitted sheet while you’re on the bed really easily - just lift the head, bending the mattress and the corners in slightly, get the corners on, then off you hop and pull the corners onto the foot - takes 30 seconds at most, and there’s no walking around or lifting the entire mattress.
