HP did have Linux desktops and laptops you could buy.
Nobody bought them. So HP stopped selling them.
It's easier for HP to just have systems that they test with Linux and if large customers want a Linux laptop HP can tailor it to suit that customer.
One of the most bizarre things I've noticed with Linux users is that they tend to reject any system being sold with Linux on it and rather go out and buy a Apple product that runs Linux like shit, a Thinkpad because of the bragging rights, or the Windows version of the Linux laptop because a few options are not available for the Linux version (invariably because they don't work well with Linux) or that the Windows version is slightly cheaper.
There is simply no reason for OEMs to market Linux systems when even existing Linux users won't be their customers.