I think for most privacy nuts it comes down to “I don’t trust them and it’s closed source. They could be hiding anything in that code.”
And then there’s the people who can’t afford or won’t spend the money it takes to have an enjoyable Apple experience. It genuinely costs multiple thousands of dollars to get into the Apple ecosystem and then it’s massively painful to get out. It’s basically just “corporation bad” because corporations are bad. The only way to be truly private is to not carry a phone at all and use only FOSS solutions.