If Apple (or anyone else) wanted to make a feature used, they can. For everyone else, if Siri is off CarPlay doesn’t work. And that’s by design.
Not the design of “ooh if Siri is off then voice in CarPlay won’t work” (warnable), but punishment if Siri is off.
Again this pattern isn’t Apple only but it’s bad everywhere.
I am sure that there was a meeting where they decided what to do when Siri was off and somebody decided (very possibly with ulterior motives) not to split the feature set - all or nothing. However I don't think the challenge they were faced with in this hypothetical meeting was an easy one.
The alternative is you open the Messages app and you can't send messages. You open Maps and you can't get directions (unless parked). Sure, I get that they could show a screen saying "Sending messages is not available when Siri is disabled" but now you're hitting error messages while driving.
Anyways, the main reason people would disable Siri is accidental activation, and Apple provides all the toggles needed to avoid that without disabling the core components needed for CarPlay.
I don't know how it works on CarPlay but when I turn my car on I have a bunch of suggested addresses (home, work, parents, recent Maps searches, etc) that I just touch-to-go. Having to use voice every time you want to navigate not only sounds unnecessary, but cumbersome.
CarPlay does not work at all if you have not enabled Siri. As in it won’t even connect.
I literally see these things every time I drive. And I work from home.