My understanding is that unless you actually keep a jet on standby (which has its own costs), it is surprisingly difficult to beat commercial airliners on speed if the clock starts ticking now. You can always board the next flight, but with private jets it takes time to a) find a free jet and b) prepare for the flight. Of course, the more non-standard journey you have, the more edge the private jet gets.
There’s also just a lot of commercial jets everywhere. Even if you kept private jets at every airport in the world you’d be out of luck if you needed to deliver two parts from the same location.
