Nothing, and so they do. Dell will sell you an XPS 13 with Ubuntu installed. Lenovo will let you select Ubuntu or fedora in some models. System76 and Tuxedo will sell you a bunch of laptops only with Linux. Starlabs sells Linux laptops. KDE sells a laptop. Purism sells Linux laptops.
Did you just assume no one sells a Linux laptop?
This looks really cool, good luck with the kickstarter Mr. Shea.
PS. I loved your Shadowdark prep videos!