launch - Why are spacecraft not air-launched from airplanes

Lots of good stuff on this topic in Wikipedia.:
Air Launch to Orbit

Air Launch

A typical rocket spends the first few seconds going straight up (almost) to get out of the atmosphere. After that it spends almost all of its time accelerating to orbital velocity. Thus getting out of the atmosphere, while hard (rocket is heaviest at this point) is really only a very small portion of the process.

The mass you would need to carry to a higher altitude is so large as to exceed the capacity of the largest aircraft ever built, let alone a balloon.

Consider the case of StratoLaunch. They intend to build a carrier aircraft composed of parts from 2 Boeing 747 (in the top 3 of the biggest aircraft commercially available. A380, B747, and C5 Galaxy are probably the biggest). It will be the largest airplane (in mass, wing length, etc) flying if it succeeds.

Even then, it can only carry a scaled down version of a Falcon (The original payload was to be a SpaceX Falcon 9 but with 5 engines instead of 9 and a concomitant reduction fuel/oxidizer load and thus mass). So the biggest aircraft yet to be built could only carry a smaller version of a medium size booster.

The primary benefit of air launch is not extra mass, but rather launch constraints. If you launch from a fixed site, you have limited launch options to differing orbits, and inclinations. An aircraft can in theory fly to wherever is convenient to hit the right orbital parameters. (Assuming there is a big enough runway for something the size of Stratolaunch vehicle within flight range fully loaded).

Currently, there is the example of Virgin Galactic's SpaceShip Two that uses the White Knight carrier vehicle, whose payload to actual orbit (LauncherOne if they actually build it) would be in the 100 kilogram range.

Pegasus, launched beneath a Lockheed L-1011 (3 engine airplane), maxs out around 500-1000 kilos to orbit.

There was a European proposal to launch an orbital payload (in the 200Kilo range) from the top of an Airbus A300.

The scaling up, that would be required just does not work.