They do because it's true and that's exactly what the law says.
Digital Economy Act 2017 14 (1):
>A person contravenes this subsection if the person makes pornographic material available on the internet to persons in the United Kingdom on a commercial basis other than in a way that secures that, at any given time, the material is not normally accessible by persons under the age of 18.
Section 23: Regulator’s power to require internet service providers to block access to material
(1) Where the age-verification regulator considers that a person (“the non-complying person”) is—
(a)contravening section 14(1), or
Go read the "commencement" section - it's actually eye-opening to do this for other laws you've heard are supposed to have drastic effects.