The AOSP dialer is not a Google Service, it comes with android.
Google phones have use a different launcher (Which might be a modified version of the AOSP launcher, haven't used AOSP in a while) that provides unknown number names, access to the voicemail-to-text feature of Project Fi (and probably Google Voice), and probably a couple more things.
BTW, from the little I know, seeing google contacts in the dialer doesn't mean that it's talking to google. Google services adds the contacts to the phone's centralized contact management and the dialer gets them from there.
..but the point is the manufacture didn't provide the AOSP dialer. They added a 3rd party dialer that you have to root the phone to disable and you need to install a non-privacy invasive dialer either by finding an apk for the AOSP version, using F-droid or using Google Play/AmazonApp services.
Google phones have use a different launcher (Which might be a modified version of the AOSP launcher, haven't used AOSP in a while) that provides unknown number names, access to the voicemail-to-text feature of Project Fi (and probably Google Voice), and probably a couple more things.
BTW, from the little I know, seeing google contacts in the dialer doesn't mean that it's talking to google. Google services adds the contacts to the phone's centralized contact management and the dialer gets them from there.