I don't think you can just look at the "number of people who are doing this travel", as those same planes are also carrying air cargo and US mail. Not everyone flies, but almost everyone in the county receives mail, cargo, or benefits from same. (It would be easier to replace cargo than passenger transport with a more efficient and comparable total trip time mode of transport if such was available.)

The reason you get asked whether your USPS parcel contains hazardous substances X, Y, and Z and why the fines for violations are so stiff is partly because of passenger airline safety concerns.