Way back in the sixties, when duvets were an exotic innovation in the Uk (they were called continental quilts then) and most people had never heard of them, I visited Germany. The beds all had enormous fluffy down quilts, in snow-white covers.
Those covers were removable, but with washable duvets with manmade fillings and polycotton covers, why do we insist on having a separate duvet cover and a separate top sheet?
Also, on the continent it is more usual to have two single duvets for a a double or kingsize bed, not one large one that is a pain to change the cover and impossible to get into a washing machine.
Recently I have been using a single manmade duvet (Ikea, white, very cheap) and not adding any cover. I can wash and spin it, and drape it over a clothes-horse indoors, and it is ready to put back on the bed that evening or even sooner. I can change the bottom sheet and the pillowcases and leave the bed open all day, and just add the duvet when I go to bed. No wrestling with the cover!
(Ikea prices - £4 to £165. Mine was called "Mysa sno" and was about £10, but they don't seem to stock that now)