Ignoring that the engines only work in air, so say its a military jet. Why is it that we don't go into space simply by using airplane technology to go up as much as possible and then switch to a rocket?
For example (although I mean just one craft, not two) in one of the superman films, a space shuttle is on the back of a 747, but instead of just being transported, it launches off the back of the 747.
Surely this uses less energy, I'm not sure, but it seems logical that a 747 (even with the weight of the shuttle on it) uses less energy to get to its maximum altitude, than a shuttle launching vertically used to get to a 747's max altitude.
What physically prevents this? Is there something about horizontal flight that is unsuitable for space flight?
EDIT: I didn't make it that clear, but if the 747 was modified so it's altitude was greater, a kind of combination of military jet and 747.