Why are piggyback launches done by dropping the carried aircraft and not by "fly-off" launches?
Its much safer, you don't kill the carrier aircraft if you drop away and have a major aircraft malfunction.  Gravity gives you the separation you need.

If you treat the two aircraft like a formation flight, which it essentially is, its dangerous for the critical aircraft to go belly up, ie blind, to the carrier.  In an emergency the first step in formation flying is to ensure separation which the critical aircraft can't do because it can't see if it's clear to descend and it may not have the ability to laterally sidestep.  Thus, a blind push on the nose could kill both aircrews.