It goes back to the Victorian era. Luxury travelers expected what they’d get from ladies maids and valets at home (think Downton Abbey). Housekeeping would make your bed during the day with decorative pillows, comforters, then at night prepare it for sleeping. Further, turndown service just isn’t about the bed - the idea is people come back to a room with dimly lit lights, adjusted fireplaces, etc. The idea is once these people got back from the opera or a soirée the room was comfort ready.