To be blunt: because they're not good enough. Even ArduPilot which is probably the best mission-flying software in the open source space, or DJI who are still ahead in the commercial space, will often need manual intervention to complete a mission.

You absolutely _can_ build an ArduPilot drone that takes commands and tries to fly a mission end-to-end without intervention, but without a real-time TX link of some form, you are in hot water when it fails.