it was always left on the bed each morning.
Yes but - Housekeepers change maybe every day. Just because the blanket was left on the bed the first morning and folded and put aside that day doesn't mean it was going to happen every day. It's kind of like saying that because the first take you wen through a toll booth, the attendant greeted you with a smile and a "have a nice day" and you expect that every single time you go through a toll booth.
As stated in a previous post, don't leave anything on a hotel bed before you leave your room for the day. There's no telling what that housekeeper needs to do while you are out of your room. Not only in your room but to the other rooms in his/her area. The room next door to yours could have had a messy party so the housekeeper needs to take more time over there which means he/she doesn't have enough time for your room.
We have one son and the first thing we taught him when we were traveling was to take all his stuff off the bed before we could leave for the day. Also, notice that most hotel beds these days have solid protection under the bed so nothing can fall under the bed and be forgotten. and housekeepers don't have to make sure all items are accounted for.