Apple could build a lot of things, but they don't unless there's considerable demand, and mechanical keyboards have very narrow appeal.

Matias isn't the only vendor and that's not the only model of mechanical keyboard they make. If you're a mechanical keyboard enthusiast you're living in the golden age of options. You can custom-build your own keyboard, pick individual keycaps, switches, and more.

There's vendors like http://www.wasdkeyboards.com that are basically "Build-a-Bear" for keyboards.