Same here. I do think it makes some sense in that case.
I'm also in the situation where I use Siri for nothing, but I want to use CarPlay. I don't use the voice control for anything, in the car or otherwise. How does it make sense to force me to have Siri enabled?
There's a bunch of parts of CarPlay which assume you can use Siri. Interacting with notifications, sending / responding-to messages, searching for things in maps, etc. Apple could disable everything that would kick itself out to a Siri-interaction for input, but that'd probably feel confusingly-broken.
> Car Play will not enable with Siri turned off (at least in my 2019 Subaru).