No because most prebuilt computers are sold by companies like Dell, HP, and Lenovo. Their bread and butter is corporate machines. They sell just enough enthusiast PCs to justify doing so, and they're typically either sold for more than they'd cost to build, or right about the same, subsidized by cheaper, mass produced parts that "may or may not" be as good as the parts you buy a la carte from Amazon (e.g. Corsair et al).

And of course it can run GOG and pirated games, but Valve doesn't make any money from GOG sales (or your pirated games) so they aren't going to care about that.