Pixels are pretty much the only Android phone that supports full verified boot with custom keys, meaning verified boot of the entire base system works no matter what OS you're booting (as long as your OS supports verified boot in the first place). They're about on par with iPhones security-wise, but Pixels support alternate OSes which is really nice.

Other Android phones often either have broken verified boot implementations (such as only verifying the kernel) or completely lack it.

The Titan M makes Pixels even better; it is a security chip which stores keys such as those for verified boot.

The security of Pixels is why security-focused mobile operating systems like GrapheneOS often target Pixels instead of say, Samsung Galaxies.

Your guess is as good as mine. Misdirected hatred at Google services privacy bleeding into this debate?

Anyway, please read GrapheneOS.org in its entirety.