It doesn't show that here either.
Mostly because NVdriver (or nvidia.o) and agpgart.o aren't always needed together (for example, if you're using no AGP), so the NVdriver module people didn't put "agpgart" on the "requires" list of NVdriver.
And that "used by" column isn't really everything that's using the module. It's just the modules that list that one as required in their module stack thingy, AFAIK.
Another way to look at it: Used By is the modules that will load this one automatically every time they're loaded. For example, my "fat" module is used by my "vfat" module according to lsmod, and all that that means is that whenever I load vfat, fat will come with it as long as I use modprobe.