Probably because diagonal is the only supplied value. That is mostly used for comparing screen sizes and little else. At least in my metric world.

You could probably measure screen sizes in fractions of blue whales and it would work out exactly the same for most folks.

Many places in europe have legal requirements about using national standard units. In Slovenia for example, you'll always see both inch and cm diagonal measurements for screens (with W×H usually only in cm).

Still it’s nice to do to give you an idea (and vice versa to imperial)