Because that would require a lot of RAM for the multi-user switch to work flawlessly. They cheap out and force the processing onto your iPhone. It's a glory square CarPlay screen ..
edit: OK, some media outlets are saying Apple has told them "developers can bring over existing CarPlay apps", but there's no mention of it in anything Apple themselves have put out publicly which is why I was confused
> It doesn’t have an App Store. Instead, Apple has a curated selection of apps, including FaceTime, Messages, and Safari. A handful of third-party media apps like Spotify are available, and developers can bring over existing CarPlay apps.