Matt Barrow wrote:

Because that would NOT make their lives easier (due to congestion on the
airways)?

FWIW, I remember Dan Brown (Atlanta Center) writing on AVWeb that he only
offered direct when his workload was low. According to him, it is easier to
keep track of lots of aircraft when they move in well-known patterns than
when they move all over the screen (even if in straight lines). IIRC, he
characterized issuing direct as a favor of the controller that makes then
pilot's life easier but not necessarily the controller's.

I don't have the link handy, but he wrote about issuing direct more than
once, so it should not be too hard to find.

Anno.