Simple answer: Because the last set of eyes to see an aircraft depart from the gate should be a trained, skilled AMT. Why? Again, the answer is simple. The AMT knows what is wrong and what is right. Is that hydraulic fluid leaking? Fuel leaking? Lav. juice leaking? Or is it just water from the airconditioning bay?
Like Hopeful states, having a mechanic on the headset also saves time and money in the event of a discrepancy in the cockpit. No slam against FSCs but if the pilot said that he had a flickering HSI or "Off Flag" or high EGT would the FSC be able to rectify the fault? No. He wouldn't. Why? Simply because he isn't trained to do so.
Also, I have seen a AA FSC push an AA MD 80 tail into a RENO Air MD 80 tail and try and disconnect the tug like nothing happened. How about the recent Alaska Air MD 80 that had an emergency landing because a FSC put a hole in the a/c with a belt loader and said nothing about it?
My point is that trained, skilled eyes are needed with machines that travel with 100+ people just below Mach 1. I do not mean to imply that ALL FSCs are like the two examples I mentioned above. They are not. But they are also not like AMTs.
Well Ken, I certainly agree that some AA FSCs have made mistakes. On the other hand, so do mechanics. Although aircraft mechanics get it right 99.999999% of the time I can clearly remember an instance at EAL where they royally screwed up. They forgot to put the O-rings on the chip detectors on the 3 RB-211s (these are engines on the L-10-11s). During it's flight, the oil in ALL 3 engines was lost and the aircraft barely made it back to MIA on one engine. Not knocking aircraft mechanics, but they are not absolutely perfect either. I and other FSCs have spotted lav leaks, water leaks, hydraulic leaks when the system is pressurized just before pushback, and other items. I AM NOT SAYING I AM AN AMT OR TRYING TO DO HIS JOB but if I see anything on my pre push walk around I report it and they call the AMT. If the cockpit has a problem during a push, which is very very rare considering how many pushes I have done, the plane goes back to the gate and mx is called. As for the Alaska incident, they hire contractor people for rock bottom wages and the one who put the belt loader in the side of the plane either didn't know of the possible consequences or didn't care. Also, the end of the belt should not have been that high in the first place.