Because they’re white. White = clean is decades worth of marketing into every aspect of our day to say life. You see white cloth, white paper, white paint and you automatically think “clean”.

The paper rolls themselves are variably clean or sterile because each company has its own level of ethics enforcement.

How it’s Made show had an episode on them, if I’m remembering properly, and their production involves washing and cooking them at high temperature, so there is that initial sense of sterilization for the most part. However, what happens after can add a lot of bad stuff. From how often they maintain their equipment, whether the packaging itself is clean, delivery and storage until sale, there are a lot of chances something nasty can hitch a ride along the way. More so if the paper is scented and attracts rodents thinking they’re getting a nice meal.

And that’s just the kitchen paper. There’s also recycled paper which tends to use all kinds of nasty shit from wherever and the recycling process simply can’t completely sanitize.

Anyway, paper towels have the perception of seeming clean because they look clean, but they usually aren’t.