People mostly think that just because a paper wallet is offline that they're already safe, even though a lot of people mess up the process by leaking the private keys while generating the wallet.
I'm amazed that after so many years, people still that paper wallet in the form of printed-out private key is the best way to store their coins.
As far as I know it's the "free" factor knowing that you don't really need a hardware wallet of some sort, knowing that all you need is a computer and a printer; and the somewhat false(or sort of misleading) sense of security a lot of guides and articles give their users. That just because paper wallets are offline(but not necessarily unleaked), it automatically means it's "secure".