Air launch does not actually provide that much benefit.
The benefits are basically, that starting at 40 or 50,000 feet allows the following:
- The nozzle can be closer to vacuum optimized on the first stage which is good.
- The launcher can fly to the equator, for a 0 degree inclination launch which is useful.
But as noted, the mass limits on the booster are huge, and rockets are much bigger than any aircraft in existence can lift. Look at the size of Stratolaunch's proposed airplane. It will literally be the biggest airplane in existence and can only lift a smaller than Falcon 9 booster due to mass limits.
The reality is that launching from the ground gets the rocket up to the 50,000 foot mark within the first minute or two. The issue with space flight is not getting high enough, rather it is going fast enough to get into orbit. So rockets typically fly mostly straight up to get out of the 'optimum' amount of atmosphere before turning to accelerate into an orbital path.
Thus airlaunch seems like a good idea, but its limits and minor benefits generally do not pay.
Pegasus was using a fairly powerful launch aircraft (L-1011) but only had a miniscule payload. Stratolauncher will use a huge launch aircraft, possibly at the limits of how big one can be, and will still have a fairly small payload.