Cause it's true. They have far less survivability than marauders for example. Sin's only have 2 defensive abilities (excluding darkness spec) and after that you gotta count on your light weight armor and some invisibility tricks.
Good part is they have quite a burst.
1 ) Blackout is 25% damage reduction. It procs immediately after exiting stealth and has a 45 second cooldown. (not to mention it combos with Force Cloak)
2 ) Force Shroud is a minute cooldown
3) Deflection is 2 minute cooldown
4) Force Cloak is only a minute and a half
Now, those are the direct defensive cooldowns. Let's talk about the indirect cooldowns. Which would be the 6 control abilities we have access to.
1) Electrocute is a 4 second stun on a minute cooldown
2) Spike is a 2 second knockdown on a 30 second cooldown (requires stealth)
3) Overload is a 20 second knockback
4) Mind Trap is a 8 second mez (Stealth required)
5) Whirlwind is a 8 second mez with a 30 meter range (1.5 second cast)
6) Low Slash is a 4 second mez on a 15 second cooldown
The last little bits that add to our ability to survive...
1) We are a stealth class. If positioned correctly you will not be the initial focus of enemy fire.
2) Force Speed is a 15 second sprint. This can be used to escape, powerful when combined with a 12 second Force Slow
3) We have a base 30% damage reduction to all AOE abilities (Smash anyone?)
I'm going to go out on a limb and say that any Assassin that complains about survivalbility is probably using the class wrong.