Today’s second glitch:
Get rid of the comma after long.
Yep, that’s it. Otherwise, this example is fine.
It’s been so long that no one knows why the walls were built. Nobody wants to leave.
Change the wording, though, and the comma makes sense: It’s been a long time, which is why no one knows why the walls were built. Not that I want to confuse you now with the difference between that and which, but there is a difference, and part of that difference is how a sentence containing one or the other should be punctuated.