Because if we don't are we really "web" developers or are we Chrome developers.
literally the same thing as asking application-oriented developers to code and test their apps for every major compiler out there.
Back in the 'olden days' we used to do that too. Some developers would claim their program ran on "Unix" when all they'd actually tested with was GCC on Linux. Sure, sometime it worked or other Unix platforms out of the box and sometimes it could be made to work, but it was still a "Linux" app, and 'we' did complain about that too (and got promptly ignored by a lot of developers who felt just like you do).
And to confess my own 'sins' the web app I'm currently working on is officially Chrome only, and it makes my life so much easier, so I'll admit I'm part of the problem.