Because I was generally going to look somewhere else for information that I needed to put on that line. Like, say, what a variable was named. Or to select something to cut and paste.
As for 100K lines, that isn't one file. That's across the project. But since a lot of that is in modules that load each other, much of that would always be in memory if you're trying to incrementally recompile.
Cheers,
Ben