The reason is, freedom, exploration at your own pace and many more stuff.
That, however, directly plays into "the world is big and expansive". I mean, that's what "big/expansive" means in terms of gameplay. But, it is NOT ACTUALLY TIED TO IT. I'll get to that in a moment.
It's not just that though. One of the reasons I don't get it is because they spend so much time building up this world... but the actual gameplay is... some of the most clunky and non-engaging I've ever seen, period, which is just about one of the worst things I can say about a game. I mean, character movement was slower than a dead frog in sludge. Combat was also slow, and very awkward. The interface was even more awkward. "Slow & awkward" pretty much sums it all up, and in the end, for all the "freedom", you were still doing the same bloody things as in every other game... dungeons and fetch quests and boss fights... it's just that in between those, you had these really blank periods of what often felt like utterly pointless travel. Sections where NOTHING HAPPENED.
Why not remove the "literally nothing is happening" bits and just keep the gameplay engaging THE WHOLE TIME?
I had that very same question about the recent Zelda, which I also hated. It was BORING. Yes, the world was pretty, blah blah blah, but as I've repeated often, you could have shrunk the game world in that by 70% and STILL kept in all of the content. ALL of it. The "big expansive world" is, to me, padding. Just padding. I compare that to the original Zelda, which STILL had a big world to explore... but the ENTIRE THING was filled with stuff to do. They didn't make it big JUST to be able to say "look how big this is". They made it big so that they could fit everything in it. Literally EVERY SINGLE SCREEN in the overworld had SOMETHING to do in it. No, I can think of two screens... exactly two.... that seemed a little pointless (I later found out they actually have secret caves in them). That game still had the utter freedom, hell, you could even sequence-break the heck out of it. You could end up in, say, the 7th dungeon (or is it the 8th?) immediately after getting the freaking candle. You went through that game YOUR way. That concept, absolutely makes sense to me. It's actually something I actively look for and I often make choices on what to buy, based on whether or not the game in question is going to let me do things MY way. So that concept is not part of what goes over my head here.
But that's not what "open world" games seem to *actually* be about. They seem more about bragging rights, to get people to buy them. As in "look how much BIGGER this seems to be than all those other games". I mean, have you noticed that ads/articles/whatever about upcoming open-world games tend to never shut the bloody hell up about just how big the game world is? They often talk more about that, then what you can actually DO in those worlds. They create all of this space AND DONT USE IT ALL.
I mean, hell, the fact that "fast travel" exists in these games isn't, to me, a feature: it's a symptom. It says "we know the travel is bloody boring. Here's an option to just skip it". If a game is literally giving me the option to SKIP parts of it, that.... is a problem.
I've seen ONE open world game that I honestly think got it right. One. Which is Just Cause 2. Like the original Zelda, it had this big world.... AND ACTUALLY FILLED IT WITH STUFF. It didn't matter where you were in that game's map. Did not matter. No matter where you were, THERE WAS SOMETHING TO DO OR FIND (usually both). I mean, literally everywhere. Even the freaking ocean held stuff to do. There were never any moments of boring travel, or nothing happening, yet it was still expansive, AND it heavily emphasized the "do it YOUR way" aspect. Overall, they got it right: Gameplay over spectacle. By not making the world bigger than it needed to be, they created something that was CONSTANTLY engaging, instead of OCCAISIONALLY engaging. And they showed that they could do this without the world shrinking too far and not being interesting to explore. There were STILL loads of areas to see. And all the way through, fast travel did not even ONCE feel necessary. Not... once. Hell, I don't know if the game even HAD that, because I never bothered to check. It simply never had any dull or slow moments despite how freaking big that game's world was (and it was quite huge... just not UNNECESSARILY huge). What's more, the actual gameplay was just FUN. Combat was fun. Driving was fun. Flying planes was fun. The grappling hook / parachute combo was BLOODY AMAZING. It didn't matter WHAT you were doing at any time: It was constantly fun (and controlled very well), yet also quite varied.
In other words, that game still had the things you claim to love about Skyrim (including game length and sheer amount of content).... but it proved it could have them WITHOUT that unnecessary padding. It proved that an open-world game CAN be done that way. If you were in a bit of gameplay that for whatever reason seemed slow? It wasn't because of the game design. It was because you, the player, were actively choosing to take your time for whatever reason. It was NEVER forced on you... but it FREQUENTLY is in games like Skyrim.
And honestly, I think LOTS of gamers don't know how or why that lack of forcing the slow sections is a good thing, because the style of design used in Skyrim, or in Ubisoft's games, or Zelda, has become the norm. Hell, it's gone BEYOND "the norm", and instead become "the only way it's ever done". Nobody even makes games with the open world theme that are constantly engaging beyond the "ooh this area is big, look at the bigness" aspect. Instead, they do everything they can to distract you from the fact that, hey, nothing is happening here and you need to do some very blank travelling to get to an actual thing.
Dont get me wrong: I dont think Skyrim is a bad game. I dont think Zelda is a bad game. I definitely accept the fact that plenty of people love them. But I think these games fall into one of the biggest, and nastiest trends that plagues the AAA side of the industry right now, and I think it really does hurt the experience.... but most will never see that, because they genuinely have nothing else to compare it to. They dont get to ever see that it COULD work differently... so they think it CANT. They think that it's REQUIRED to have that "freedom". That's why the boring parts may not seem as boring to you... you've never seen how it COULD be done, because there's been almost no examples, and the genre is firmly entrenched in that "norm". And players accept (and perhaps dont entirely notice) the forced boring moments as something that there is no way to design around it.
