I'm curious why is -e is what I usually see for file existence checking and not -f?

I'd say it's probably because -f can be a little too restrictive:

$ perl -le 'print -f "/dev/null" ? "IS" : "is NOT", " a file"'
is NOT a file
[download]

but it's also true that sometimes -e gets overused when a more restrictive check would have been better, for example at least checking ! -d.

Update: Just to clarify what I meant with the above example: If a user is supposed to supply a filename to a program, then in a lot of cases the program shouldn't reject /dev/null and just work with it as if it was a regular file. Of course there might be cases where a program would want to accept only regular files - but then again, -f also returns true if what's being tested is a symbolic link to a file, so one would have to check for that specially too.