Apple and Samsung are (primarily) hardware companies, whereas Google is an advertiser. If your income depends on hardware sales, longevity/reliability is an important factor for your long-term brand reputation that you can’t afford to sacrifice.

Google-branded phones are essentially just another avenue for them to push Google Play Services, one where they can guarantee the platform experience they envision developing vanilla Android. I think they also serve as reference hardware, but the custom tensor chip on the most recent pixels sorta eschews that… unless they are pushing the idea of putting a TPU on Android phones.

Its similar to why NVIDIA(designer/manufacturer) doesn’t offer as good customer service or warranty on their graphics cards as EVGA (hardware/packager) does.