Probably because it's unnecessary. If you want direct just file direct.

Out West direct for the entire flight is not usually possible with
non-turbo aircraft. What usually happens is when you get half way in
the middle of no-where on an airway, the controller will send you
direct to some intersection because he knows you are now far enough way
to avoid the mountain rather than have to continue to fly down the
airway until you get to the next intersection. Besides, I can't
remember the last time I was actually given the route I filed. I've
tried to second guess the routes in the Bay Area and L.A (even the
central valley) but they seem to change on a daily basis (probably
depending on which way each airport is landing jets). When flying into
SoCal you usually get an "updated" routing about 1/2 way down. That
routing is usually longer than the amount of space you have on your
flight plan (bring extra paper to write it all down).
-Robret