Hotels comforters are laundered, on average, every 3 months. I know this because I used to work in the hotel industry for a while. One of the hotels I worked at had comforters and then underwent refurbishment and had duvets after. Many hotels are going to duvets because they are cleaner and easier to launder.
I do blame POR partly for this. Housekeeping issues aren't like bus issues where they can occur at any resort since drivers' routes are moved around. To my knowledge, housekeepers at one property generally only work at that resort. The 8 days we were at POR, we had the same housekeeper every day and she was sub-par. When we first walked in the room after checking in, there were crumbs on the rug by the AC (stayed there all week. vaccuum? nah!), rings from drinking glasses on the dresser, finger prints on the mirror from someone else. No major issues, but clearly the room wasn't cleaned well.