If you can traverse an "open world sandbox" in seconds by, say, fast travel a la elder scrolls, then what is the purpose of that open space? There's no point in having it if it's just there for scenery and scope if you can't do anything with it.
Morrowind, for example, had a huge amount of open space, and yeah, a lot of it was fairly empty, but there was always some random little caves and dungeons and side quests you could do that had no bearing on the main story but broke up the monotony.
Compare to Oblivion, where everything was based around using the various towns as "hubs," and all you did was fast travel to the nearest hub, walk to the quest, do it, then fast travel back to the quest giver. They had a huge empty space, but the player didn't get to see basically any of it, because the "click to teleport" nature of the fast travel made it irrelevant.
Open worlds are only interesting if the player is forced/encouraged to see it, to explore it. How much crap did WoW get when they made flying possible, and how much did they have to do to force players not to use it until it was irrelevant, or to remove it in new content, etc.
Players will almost always use the quickest easiest mode of travel. If that mode of travel involves skipping content, that content may as well not even be there.
I'm much more a fan of tightly designed, smaller world's that are jam packed with content that is relevant and interesting, rather than just making an open world and saying "we've got a huge open world!" No you don't. You have a vast wasteland that no one cares about and no one wants to see except when they're forced to, to get to their next objective (Fallout games, assassin's Creed, so on).