I think it's because almost all web design happens on computer screens, not mobile devices. More important, all web designers show their bosses their creations on a big computer screen in a conference room--the bigger the better so everybody can go "oooh!" and "aaaah!" and the designer can get a big fat raise.
I expect it's fairly rare for the employer of a web designer to give a shit what the site looks like on a phone, which means the designers don't give a shit either.
