And how many campaigns are run at that level?
In any case, bumping into the "big names" of any setting once you're level 17+ or so shouldn't be too surprising - in Planescape for example, at that level characters may very well have gotten the notice of the Court of Stars/The Five Companions/The Hebdomad or some of the Archdevils/Demon Lords.
What I was responding to was the suggestion in several posts that characters of low to medium levels in the Realms are stripped of their agency because some high level NPC is going to inevitably swoop in as a deus ex machina and take out any true threat the characters may face, which is simply not the case for any DM that doesn't want that to happen. Simply don't use said NPCs, or use them sparingly as background flavor at that point.
Yes, the characters may encounter them at higher levels, but by that point those NPCs are now more peers than superiors, and anything powerful enough to threaten the characters now threatens the NPCs as well. And even then, they are avoidable if a DM wants - if a threat escalates quickly, there might not even be time for the big gun NPCs to get the news; heck, they might be on another plane entirely at that point. Or if a threat is big enough over a widespread area, said NPCs may have to concentrate on some areas while the characters deal with the situation in others. After all, if demons start pouring out of gates that are suddenly appearing without warning all over the Realms, Elminster will be busy dealing with the one that opened in Myth Drannor and Laeral with the one that popped up near Waterdeep - and are my group just going to sit around and watch Baldur's Gate get wrecked assuming that some high-level NPC is going to deal with the issue? Not if they are heroes they're not!