And police are exempted because...?
I can only speak to fire and police retirements, but they are really expensive in California. LA fire gets something like 90% of their high three after 20 years and 100% at 25 years. That includes the ability to work lots of overtime to spike the last three years earnings. Why would somebody keep working if they made more by not working?
The reason a lot of feds keep working until they hit mandatory retirement age is because they make a fraction of their working salary in retirement and pays to work longer.