There's different variables in place this year for Murray compared to Parker and Kawhi's rookie year.
When Parker was a rookie, Spurs had just let Avery go and Pop was never big on Antonio Daniels. Spurs had just lost their one play-maker on the perimeter with Derek Anderson going to Portland. Parker was forced into the starting role due to high demand -- Pop really had no choice but to throw Parker in the fire. Murray doesn't have that situation in front of him with Parker and Mills ahead of him. Spurs don't have that desperate need right now -- this year. As for Kawhi, he came in when Spurs desperately needed his size at the wing in the rotation. Jefferson was the only true SF on the roster and he had been a disappointment for the most part. Both Parker and Kawhi came into desperate situations for their position, it's not the same with Murray.
Also, Simmons has played the back up SF this year -- not SG. It's clear the Spurs haven't had Murray as a SF or a wing this year. All of his minutes (sans maybe 5) have been at the point guard.