They pretty much ported Skyrim to every platform known to man.

It is kind of surprising Fallout 3/New Vegas/Oblivion hasn't received the same treatment... But then again, they have aged extremely poorly, visually I guess.
Same with Morrowind.

Fallout 3/New Vegas/Oblivion would still look OK in 1080p and not that difrent compared to Fallout 4 and Skyrim, but Morrowind is totally different story.

Fallout 4 wouldn't run on the Switch.

Yeah, that's the main thing.  Skyrim holds up decently well with new textures and lighting and such.  Oblivion...no, it would need far more.  Aside from engine stability and optimization, it would need a ridiculous ammount of new geometry.  The trees use billboards for goodness sake, that hasn't been an industry standard for folliage for the better part of a decade.  And of course the character models scream early 7th gen struggles.

Oh no they would look very, very different and much worse.  Trust me, spent over a thousand hours combines across those three and hundreds in Skyrim.  Those three would need a massive upgrade to be passable.

It could be done something similar like it was done for Skyrim remastered, higher resolution, better lightning, better AA/AF solution that gave crispier textures...and Fallout 3 wouldnt definitely look that much different than Skyrim remaster.

Oblvion is 2-3 years older game than Fallout 3, but despite that Oblivion looks OK on Xbox X that actualy just upscale game and its not full remaster.

https://www.youtube.com/watch?v=Qh8mRYHQp6g

When NS user base will be large enough, more games could start being developed with NS as entry-level HW, but for existing games that were developed on more powerful platform, a port, if possible, will happen only if the predicted sales will be large enough to justify the costs. In some cases NS HW specs aren't an obstacle, but, as others wrote, if the game didn't age well, or if potential buyers are too few, a port won't happen, unless Ninty pays for it.
A possible solution to make some ports happen could be crowdfunding, it could also be a way to see if NS gamers really want those games.

New Vegas looks ... okay.  It's mostly rock, anyway.  Fallout 3 looks nowhere close to Fallout 4.  Fallout 3 would need a LOT of work if they moved it to current platforms, not just visually, but internally.  It just does not hold up very well.  However, I do think a rebuild would make enough money to justify the expense.  The problem would be having enough resources to spare while working on several other games.

I also would not be surprised if Starfield does not come to the Switch.  They started work on it directly after Fallout 4 ended, which most likely means it's the same engine.  I think that means it's built for PS4/XO in mind.  A lot has to do with when Nintendo started seriously talking to Bethesda.  I'm not sure if we know the timeline (I don't).  Bethesda gave Nintendo an absolute tongue-thrashing in 2013 for not communicating.  Initial pre-production on Starfield probably started 2015 as Fallout 4 was finishing up.  When did Nintendo contact them about the Switch?

Elder Scrolls VI is a different beast.  Signs point to a new game engine.  Howard made a comment that it would sound like they "don't even have the technology" for what they are working on.  That has next-gen written all over it and they probably have a rough estimate of what Sony and Microsoft are planning.  Nintendo is notorious for not sharing that kind of information--but, then again, I think Bethesda has proven that they can keep secrets well. 

Good.  Fallout 4 was a disappointment.  Give use New Vegas or Fallout 3.

That would be pretty great

Now add some Wolfenstein, Doom, Fallout and Skyrim Collections pls