The mass fraction/payload from air launch and SSTO concepts is inferior compared to traditional staged rockets.
Getting your rocket up to .8 Mach and 50,000 feet seems like a "head start", and in theory it is, you're starting out close to where a traditional booster has just fought its way to max-q, and it can use an engine bell that's optimized for high-altitude and vacuum.
However, actual payload savings from air launch are around 5%. Not that much. You lose significant energy and altitude in the drop, and any wings or shuttle-esque lifting body shapes to convert the forward flight to vertical from that drop is dead weight once it's thrusting upward for orbit. Like within 15 seconds.
To compete with the payload a Falcon 9 can throw to LEO, much less a Falcon Heavy, or the BFR/Starship, you're now talking a carrier plane the size of a SHIELD hellicarrier from the Marvel Avenger movies.
Stratolaunch dropping a Pegasus II would put 13,500 lbs. to LEO. Falcon 9 puts up to 50,000 lbs. to LEO. F9 heavy, 141,000 lbs.
And the "operates like an airline" savings and flexibility is generally overstated. The Stratolaunch when loaded needs a 12,000 foot runway for takeoff. (Not even counting runway elevation) So it's not as if it would operate from just any runway, so it would be limited to the various Shuttle runways and emergency alternates. And there the issue of which ones would even allow it in the first place.
Further, the "airline savings" are further curtailed by the large custom carrier aircraft. Divorcing yourself from the common commercial airline and freight logistics train and maintenance procedures.
So that theoretical 5% increase in efficiency air-launch gets you, it's an awful lot of rigamarole to get it, it's easy to lose it in other unforseen costs.
The one advantage of air launch is that you can hit most any launch window, and any orbital inclination. Although SpaceX doesn't seem to have any big difficulty with it from fixed launch sites.
