BetaFlight doesn't deal with over-the-air bits, it just receives PWM/PPM/S-Bus/whatever signals your receiver provides. There is no point to have encryption in firmware, because connection between RX and FC is hardwired and can be trusted.
Lack of OTA frames encryption, as far as I can tell, is mostly due to legacy reasons. In DYI FPV there are only couple of transmission standards, most of them using 2.4GHz FHSS or some CC2500 clone so you can mix-and-match transmitters and receivers as you wish. If you use custom TX/RX devices, you are pretty much locked in to that specific vendor. Also, designing a nice transmitter UX-wise requires quite a different skillset than designing nice transmitter RF-wise, so manufacturers tend to choose off-the-shelf RF modules.
The threat model for most FPV pilots (either hobbyists or people in Ukraine) doesn't really include hijacking of the air link. It's trivial to just shoot something down with interference, sometimes inadvertently.
Pretty much everyone in FPV is now using ExpressLRS, which is an open protocol. If you want an encrypted air link, then the best option I'm aware of is the proprietary TBS Crossfire protocol.
Lack of OTA frames encryption, as far as I can tell, is mostly due to legacy reasons. In DYI FPV there are only couple of transmission standards, most of them using 2.4GHz FHSS or some CC2500 clone so you can mix-and-match transmitters and receivers as you wish. If you use custom TX/RX devices, you are pretty much locked in to that specific vendor. Also, designing a nice transmitter UX-wise requires quite a different skillset than designing nice transmitter RF-wise, so manufacturers tend to choose off-the-shelf RF modules.