Because if you are ever arrested with your phone, even if it is encrypted, you may be forced or coerced to unlock it.

To add, if you do encrypt it, always use a password or passcode, because they cannot legally compel you to divulge the content of your own mind (at least in the USA... for now) while if you use a fingerprint or facial scan, they are totally allowed to force you to show your face or use your fingerprint to open it.

It would be really helpful/cool if people got into building LoRa swarms of small texting devices for such events.