This reminds me of a private conversation I had with my then 14 year old grandson. His personal hygiene was fine. He was responsible for putting sheets on his bed. He didn't. He just slept on the mattress cover. When I visited, I went into his room and was almost knocked over by the smell.
He, of course, had gone nose blind to it and couldn't smell it. The mattress bad was badly stained from sweat and body oil.
So I asked him how long it had been since it had been washed. He couldn't remember, and wanted to know what difference it made anyway.
I explained that washing the mattress cover and sheets is the preferred method of keeping mattresses clean, and told him that it is much cheaper to replace old sheets than a mattress and box spring.
He truly didn't know why sheets were used. Now that I think of it, no one ever told me why they were used, it's just something we had to do religiously every week or so. So we washed his bedclothes and he put sheets over the mattress pad. And he probably stopped doing it immediately after we left!
On a similar note, my 13 year old nephew was excited because he purchased a pair of shoes that had socks in them, so he never had to wear socks! Similar response to sheets - socks absorb sweat and are easy to wash. The socks in his shoes also absorb sweat and can't be washed easily. That could make his shoes stink and people might start avoiding him - especially girl people! That was an "aha" moment for him!