First I've heard of that genre but I'll take it. As long as they're not called RPGs anymore, because it's insulting to the genre.
Your avatar character is from an immersive sim.
Imsims are basically games that combine simulation elements with role-playing elements. The core idea behind it is that, well, players are in the game's world. That's what drives everything. Bethesda's always focused more on creating a world than the people of that world, however, and they've focused on making broad experiences, where everyone can be anything, rather than specializing.
Looking Glass (y'know, the greatest developers of all time) created the genre with Ultima Underworld, and from there went on to games like Thief and System Shock 2. Spector left them and went on to create Deus Ex, which was heavily influenced by Harvey Spector. Since then, we've had games like STALKER, Bioshock, and so on and so forth.
Back in the day, Looking Glass had some sort of relationship with Bethesda, who, among other things, created Madden. Looking Glass were the guys who actually made the series popular, but that's kind of a story for another day.
Anyways, The Elder Scrolls came about, as I understand it, when Bethesda went "holy fuck, Ultima Underworld is the greatest fucking thing ever; we have to do this!"
So they've been developing games in their own vein--their take on Looking Glass's concept (and part of that was to create big, open worlds, rather than focus on the concept of designed experiences like Looking Glass). They've got some ex-Looking Glass staff as well, iirc, like Emil Pagliarulo. He was, iirc, the guy responsible for Oblivion's Dark Brotherhood questline (which, I am told, is the best part of the game), as well as Fallout 3 and the Dark Brotherhood/Thieves' Guild questlines in Skyrim.
Anyways, yeah, Bethesda makes games that are sort of the... evolution of the RPG. Basically, they're what RPGs have been trying to be ever since they were war games--that is, simulated adventures.
Of course, when talking about evolution of gametypes, one doesn't always mean that evolution is an improvement (see: 3D vs 2D platformers--both are completely legitimate forms of expression, but 3D is an evolution of 2D), and I don't mean that here.
It's just a different kind of game, and to go into their games with the expectation you'll be playing an RPG isn't really accurate. You'll be playing a Looking Glass-influenced game that leans a bit more heavily on role-playing elements and open world nature than emphasizing simulation and design.
That's why I take issue with the suggestion that Skyrim is "dumbed down." It isn't. It's just moved the numbers out of view as best it can, trying to emphasize the act of being within a living, breathing world as best they can. It's trying to move away from being an RPG, or at least, the traditional, limited definition of RPG (because really, a role-playing game, at its purest, would be one where you take on, in body and thought, the form of anothe rperson in another world--the holodeck).
They're trying to create virtual realities.
So
to answer your question from elsewhere, the one that asked why they're so popular, it's... because they've done one of the most important, powerful things that video games do: they put players within a living, breathing world.
Could it be better? Absolutely. Games like Far Cry and Dishonored do the whole "sense of place" thing better. STALKER decimates their games in terms of atmosphere and artificial intelligence. Obsidian has better narratives (interspersed with really shitty writing) and quest structure in their games. Crysis has better graphics.
But nobody gets the idea of open, free world as well as Bethesda.
That's why they are, and will continue to be popular. That's where their power and appeal comes from. They're fun worlds to
inhabit.
And that is why I will be there, day one, to buy whatever game they release next.
That's also why New Vegas wasn't as well received--it's
not a fun world to inhabit, it's a fun RPG to play. The use of the first-person, immersive perspective was wasted by Obsidian. They really fucked up horrendously, because they had no idea what they were making a sequel to (and yeah, I get the irony of what I just said, because the people on that team created Fallout, but the modern expectations--and Bethesda's, were for a more immersive game like Fallout 3). They made another classic Fallout game, but failed to make use of the camera perspective and engine they had to use. It'd be like someone being told to make a Metroid Prime game, and making a first-person Metroid game--but having players fundamentally play it like a 2D metroid (jumping up and down, moving only two directions, etc).