Presumably they wash though, probably after getting out of bed.

And before crawling into it.

Either way the duvet will accumulate grime at night that someone should be able to smell. I wash and change my bedding weekly and am disgusted by the low frequency others are reporting. Who enjoys the texture of besweated beddings? I can certainly tell the difference and I love clean sheets and hate stale ones! How do people not feel or care about this sensation?? Especially if you have sex on sheets that you don't wash for weeks. Holy ew.

It is certainly strange when you find out that other people have different tastes and viewpoints to you.

I change it every two weeks, but I sleep in a t shirt and underwear, and I don't sweat too much unless I'm sick.