Many of these systems are using the amateur radio service bands. Often 5GHz for video, 2.3-2.4GHz for control and telemetry and less frequently 1.2GHz, 900MHz and 70cm are also used. The FCC doesn't allow encrypted traffic on amateur bands except for specific cases such as satellite telecommand. I suppose that has discouraged adoption of cryptography.

What are commercial operators doing? Another limitation of Amateur bands is that they can't be used for commercial purposes, so licensed commercial drone operators must be using commercial bands where cryptography is permitted. Don't know myself; haven't looked into it.