Honestly I figured this was the case everywhere. The sheets are the clean part. I also wouldn't expect the couch cushion covers to be cleaned each guest.
Used to work at a hotel that did this too. Ask for the room to be spring cleaned before your visit. You might get charged slightly more, but you won't be sleeping with a dirty duvet that hasn't been washed in almost a year.
This has been a fairly open secret, or at least it has been in my family. My father traveled a lot and we would always throw those on the floor immediately. I assume nowadays many of the bigger chains do clean them but I still wouldn't trust smaller places and even smaller chain-owned hotels.
Plot twist: the Hotel did change them and all colleagues did too. Only OP learnt it wrong and despite knowing it is shitty OP didnt bother to do better himself bc he thought everyone did this