There are two immediate and potentially valid answers: "They don't" and "money."
Skyrim is popular, that's not a huge surprise to anyone. It's not uncommon for The Elder Scrolls V to hover between 18 and 25 thousand concurrent players for the base game and 11 to 22 thousand for the Special Edition release, and that's only on Steam. Those numbers may not seem like much, but if pooled together as a group of Elder Scrolls players and adjusted accordingly, Skyrim would currently stand close to the top 10 most popular games when it comes to active players.
That's something to be impressed by, considering the game came out over half a decade ago. Other popular RPG releases from 2011, including Mass Effect 2 and The Witcher 2, struggle to break the 1,000 player mark, despite being popular upon release.
There's something about Skyrim that keeps it relevant for players long after its initial impact. It's easy to presume Bethesda is doing nothing but trotting an old horse out to pasture well after it should have been retired, but there just aren't many current RPGs that claim the same retention rate.
Aside from the launch version of the game and its DLC-inclusive Legendary Edition, Skyrim hasn't received all that many ports. The Special Edition of the game only came about as part of a practice project, porting Skyrim to next-gen consoles in preparation for the then-impending launch of Fallout 4. The project resulted in several enhancements to the then-venerable game and, if you owned the full PC version prior to its release, was given away for free.
In some ways the Special Edition has superseded its predecessor, given the enhancements to the game engine and its ability to take advantage of more available system memory. In others, it's taken a step back by hampering compatibility with some of the game's most popular mods, due in no small part to the changes made to the game's code during the development process.
With the updated re-release sorted, that leaves the Nintendo Switch and VR ports of Skyrim on the table as the next two entries in the "unnecessarily re-released" category. In some ways, the Switch take on the game might not add much to the mix aside from spreading Skyrim's reach to players who only play on Nintendo consoles. The VR edition, though? That one feels different.
Virtual reality has been the dream of gaming enthusiasts and even the public at large for decades. Interactive entertainment isn't a new concept, but new technology is allowing us to remove some of the barriers between ourselves and our digital worlds to better immerse ourselves in experiences that transcend conventional control schemes. Valve, creators of Half-Life 2, want to port the flagship game to virtual reality-capable platforms in a move that might seem odd, considering the game's age.
On the other hand, porting a game that already exists to a new scheme of control is often, but not always, simpler than creating a game from scratch. VR needs strong titles to succeed, but developing new games for a rapidly-developing landscape can be costly and somewhat financially dangerous, cutting off potential developers at the knees.
By bringing established titles into the VR landscape, much of the risk of initial development is sidestepped. Costs are lowered. An established fan base can be brought into this new medium, giving them a chance to play something they already know they enjoy, and gives VR developers a reason to step outside the box and try something new.
Now if they re-release Skyrim on the N-Gage and the Dreamcast? Then you might have to assume there's something less altruistic at hand.