How do you envision this working on the product in question? When are you ever making arbitrary DNS lookups in a Chromecast?
Seriously take the tinfoil hat off for a minute and think rationally. Google owns the entirety of the software on the device, and all connections to & from it. There's nothing they gain in terms of data harvesting from hard-coding their DNS here. There is no user input in play at all here. What are they going to harvest from a device that only ever does DNS lookups for their own hostnames?
If this was happening on Chrome, or Android, or something where user input & interaction was actually a thing then sure. But this is a goddamn Chromecast. All it does is watch YouTube and similar. How in any way, shape, or form can those DNS requests in any way help sell ads?
i.e. I suspect Google force their own DNS so that one cannot so easily use e.g. PiHole to filter out DNS lookups for servers that stream e.g. YouTube adverts.
?
Well... all of the benefit to the user. Google doesn't get to use your DNS requests to sell ads.