Bethesda has always operated like that though. And it's been immensely successful because they've had a monopoly on 'comfy shallow sandbox rpg', which is an off-shoot of the immensely popular 'shallow sandbox' game design. The formula comes down to giving the player tools and mechanics for their own fun and enjoyment, without anchoring them to things that are actually punishing. A shallow sandbox. Elder Scrolls (especially post-Morrowind), Fallout 3 (and NV to a much lesser extent), GTA, Saint's Row, Watch_Dogs, Far Cry, Crysis, Assassin's Creed, Just Cause, Dying Light, etc etc.
Well, while I do agree that Bethesda has settled themselves into a niche when it comes building their type of games, I don't think it has been their policy to play releases this close to the vest, at least in the past. I remember countless hours of promotional videos where the Beth devs walked through systems and demonstrated physics or NPC behaviors or the "Ambient Quest" features, all well ahead of release of games like Oblivion, FO3 or Skyrim. It seems they are being very quiet about this release, which is interesting. Maybe it is hangover from the No Mutants Allowed blowback, maybe it is from seeing some recent developers get burned by pre-release statements or videos, maybe there's something hugely wrong with the game they are trying to fix before they show the world... who knows? Thing is, it's not how they have done their previous games, so that change in pre-release tactics will always garner discussion and speculation.
Some of the most successful games and franchises in the current AAA arena is built on that concept. It means that gamers still fundamentally enjoy playing games as opposed to the pretentious walking simulator experience that progressive media is trying to push onto people, but that 'dumb fun' is still a key component of what makes a game popular. Nothing wrong with that, but I get really spergy and triggered as it relates to open world RPGs. Witcher 3 and to a lesser extent Inquisition have done a good job in breaking the false dichotomy between non-linear open world games and quality writing/roleplaying that Bethesda and BioWare have built up in the minds of gamers since the days of KotOR and Morrowind but there are plenty of people that look at open world games and scoff that it immediately means that the writing is crap or disposable, because every bloody game that follows a similar design formula in the current industry is exactly that.
If we take a step back, I think it may be a different problem than "sandbox games have weak stories," but maybe something more along the lines of "sandbox games have problems TELLING stories." It's more a matter of HOW the story is told rather than the story itself being weak or strong. With more text based design, a lot was left to the player's imagination and the words themselves stood on their own. In the voice acting world, the voice acting was swiftly coupled with in-game animations, which introduced a whole new spectrum of difficulty in communicating with the player, as wooden animations became nearly distracting from the words and presentation itself. Then comes the free roaming camera, where a player could be having a large scene play out with someone like the Jarl of Whiterun and instead be trying to see if they can see an apple on the table for them to steal later. The delivery falls flat because the player is not engaged in the conversation - they have more points of stimulation than the "important" content underway will capture.
Which, ultimately, leads down the road of cinematics and carefully crafted scenes to tell stories, which both is exponential in cost with something like a sandbox game, as well as threatening to the RP experience of crafting, creating and controlling a character of the player's own design. And we are now seeing this with Bethesda - they realize they can tell the same story and it be perceived as better because the character will emote and interact with NPCs (whether the player wants them to or not). It's not the Bethesda could never tell a good story... it's that they could never tell it in a way that their format was conducive to. Which is a shame, because that means all AAA RPGs will be headed into the realm of large, open-world games with cinematic story-telling and voiced, more pre-set protagonists.