It's an option. AOSP isn't identical to OEM-distributed ROMs, but it's certainly a great basis for private OSes like CalyxOS and GrapheneOS. For individuals that are serious about privacy, there aren't any options to compile your own iOS with Apple services disabled.

I'm not saying that the AOSP absolves all of Google's server-side behavior (or even that it proves they're benevolent; neither company is). My point is that Google presents a realistic threat model to their users, that takes them seriously and even provides escape hatches for any potentially concerning features. iOS presents a comparatively cartoonish outlook that relies more on the strength of their marketing than the self-evidence of their security. Apple's position is indefensible but claims to be altruistic; Google's position is honest, so much that it treats themselves as a threat.