The highest altitude recorded for a jet airplane is just over 100,000 ft in round numbers. Too attain low earth orbit you would need to reach about 100 miles in altitude or about 520000 ft. As you can see a jet would only take you about 1/5th the way up and you would need to switch to a rocket propulsion due to the lack of oxygen. My guess is that switching from regular airplane/jet to rocket propulsion is an additional complication which might make it more expensive then just using rocket propulsion.

The highest altitude recorded for a jet airplane is just over 100,000 ft in round numbers. To attain low earth orbit you would need to reach about 100 miles in altitude or about 520,000 ft. As you can see a jet would only take you about 1/5th of the way up and you would need to switch to a rocket propulsion due to the lack of oxygen. My guess is that switching from a regular airplane/jet to rocket propulsion is an additional complication which might make it more expensive then just using rocket propulsion.