I searched on the subject, thinking it could probably be cheaper to fly the spacecraft aboard a plane and then launch it from altitude. The only thing I found was this Wikipedia page, but it only enumerates advantages over ground launch without saying why it's not the traditional way of doing things. Knowing the X-15 was tested in the 50s, if it was the better option, it would be known since that time.
I think one the reason may be limitations on the payload for the plane to takeoff and fly to a sufficient altitude, but it shouldn't be a problem when sending astronauts on the ISS, for example. Any idea on this?