Choose all that apply.
And the obvious, but accidentally omitted possibility: You know what shelf liner is, but choose not to bother with it.
I went to a decent kitchen shop today today to buy shelf liner, but they said they'd never stocked it, in all their years of operation. Am I looking in the wrong kind of shop? Am I looking in the wrong country? I've never installed it myself, but it's been a staple of most decent, owned-by-user, long-term kitchens (and often bathroom cabinets and drawers) I've experienced in my life.