One does not simply take out a SAM system with a cruise missile, especially when that SAM system can also target the cruise missile. So how do you get a cruise missile to launch from the right spot where the SAM system radar can’t see it coming?
Sure they are - the s300 system for example can be redployed quite rapidly & some like Buk have the radar even integrated with the launcher.
Also only the radar really transmits, so even if you hit it the, separate launchers might still be intact & guided by another radar if you don't destroy them.
This is why modern remotely guided middle strike drones are so dangerous for SAMs - they can basically search an area for all the SAM system components and destroy them all or most of them. Potentially much more effective than a homing missile hitting just the radar at maximum.