I know you’re joking, but most fire apparatus don’t have keys at all, and those that do usually just have them in the ignition all the time. You don’t want to be searching for keys when you need to get to a fire.

Security pretty much relies on them being in a secured bay most of the time, at least one crew member being close by when out on incidents, and requiring some training (or at least a good amount of time) to figure out how to start one and get it moving.