Unfortunately Skidace and Raven Crowking, you ignored the other half of what I said.
Yes, certainly, you can have ongoing, overarching plotlines in a sandbox campaign. However, if you are "free to ignore them" then they are hardly threatening the entire realm are they?
That's what characterizes high fantasy - that threat to the entire realm.
So, yeah, sandbox campaigns could have overarching plotlines, but the fact that the players can ignore them with no consequence (at least no personal consequence - like Sauron winning and everyone being killed/enslaved) means that it's not high fantasy.
Thus, sandbox does not lend itself well to high fantasy.
Sandbox, otoh, because the players can ignore whatever they don't feel like persuing, fits an episodic campaign much better, since you have no reason to follow a particular plot threat, other than personal interest.
Buffy's plotlines were contained to a single season and besides that, I'd hardly call Buffy episodic. It was serial. You started each season with a large plotline and probably half a dozen subplots that would take the entire season to resolve. How is that episodic? ((To be honest, I only really watched Buffy from about season 3 onwards, so maybe the first two seasons were more self contained, but, most of the series was certainly not episodic.))
Star Trek:TOS was purely episodic with almost no carry over between episodes. ST:TNG carried some over, the Borg for example, but, the vast majority of episodes were completely self contained. DS9 was serial. Voyager was more episodic although, obviously, it had one large plotline - trying to get home.
The new Dr. Who is almost entirely self contained. One episode does nothing to inform the next episode. Other than the Bad Wolf plotline of the first season, which was simply a series of foreshadows, a theme that was repeated in the last season, what carries over from episode to episode? Character changes I suppose.
Anyway, I've wandered far afield. You guys are arguing only half of the equation. Can you have sandbox with long term plots? Sure. But, high fantasy is characterized by world threatening events. Sandbox isn't.