DPS Guardians don't enjoy the survivability you seem to think they have.
A Vigilance spec Guardian gets Saber Ward, Enure and a 4 second, 20% damage reduction after Force Leap.
A Focus spec Guardian gets Saber Ward and Enure.
Sentinels, regardless of what skill tree they sink their points into, get Saber Ward, Rebuke, Force Camouflage and Guarded by the Force. I play both Sentinel and Guardian, and without a doubt, my Sentinel has more survivability than my Focus-spec Guardian (so long as my CD's are available, of course). Obviously heavy armour plays a factor, but it doesn't provide as much protection as some people may think.
If I sink my points into the Defence tree, then yeah, I'm a pillar of stone. Unfortunately, I also hit like a pillow.