That's unfair to say. I for one rather like windows and microsoft, even though i don't agree with all their decisions. It's become 'cool' to hate on them for their success and for the fact that they have a tendency to work around their largest market (which, fun fact, is NOT people who actually know what they're doing) but on the whole they've consistently delivered relatively easy to use products which don't rely on all the brute-force restrictions of Apples idiot-box programming.
Linux is something of an elite OS which only works if you know what you're doing. The vast majority of computer owners still have no clue what they're doing (which is, ironically, responsible for Microsoft's latest fiasco with Windows10 upgrades) and until the entire developed world is populated by computer literate people, Linux is going to remain an insignificant portion of the market.
There's no positive PR in supporting an OS that very few even know exists, let alone use, especially if you're trying to use that support as a giant middle finger to the systems which, you know, actually have a big enough share of the market to earn your investment back.
Linux has struggled to maintain a foothold for more than two decades, and has actually become LESS popular over the last 5 years as a dexktop OS. It's not worth the effort.
Already done. Fallout 4 and Skyrim Remastered use 64bit engines. The limitation was the last generation of console, and the problem has been resolved.
Won't happen. It's a poor use of time because it basically requires making two games, and frankly, it's becoming a non-issue. Microsoft in particular is quickly breaking down the distinction between Console and PC, and within the next 5 or so years has the goal of near total cross compatibility of systems. Consoles are becoming low to medium end computers, rather than distinct systems in their own right.
Even then, the time involved isn't really worth it. Most of the problems people have with their games on PC has to do with the memory limit of the 32bit engine, which isn't an issue anymore. Beyond that, it's a problem with Bethesda, not the system they build for, and making them build the game twice isn't going to solve that problem.
There ARE some problems with their attempts to port to the keyboard and mouse interface, but it has more to do with the fact that they won't commit to a single control scheme than the porting its self.