Websites are not art*

They are communication mediums. Either delivering communication effectively to the user, or to let the user communicate effectively with others, or do so bi-directionally.

Or they're tools to get work done - maybe even make other art.

In all cases functionality is most important - which is not incompatible with aesthetics. Aesthetics and usability are vital too.

But it does constrain somewhat the variety of your medium. And that's okay.

As LitFan said - it's the same reason cars and smartphones tend to look the same as well.

* Except when they are, and when they are they should be appreciated as such and graded as such, and there are such websites, but they are by far the exception.