First of all, it's not my decision - I tend to agree, but in fairness, it's not as simple as we'd like it to be.
A modern trunked system is essentially passing digital data over an IP network. Some parts of that network are RF while others are not.
Some of that data is protected PII.
Put yourself in the shoes of the radio sysadmin for a bit. You can implement an unencrypted dispatch talkgroup and one or more encrypted talkgroups for e.g. medical info, and then you have an education/training problem in which "the way we've always done it" is going to keep rearing its ugly head. Alternatively, you can make everything encrypted and everybody (fire, law, etcetera) can keep doing things the way they've always done it and you can sleep easy at night knowing that nobody's PII has been compromised because of your radio system.