Notably, the only other serious competitor in the space is also not open source. Sure, you can probably carefully construct a phone from only FOSS, with some compromises of course. But this is unfeasible for regular users, who have for all practical purposes only two choices. And those same users are unlikely to go for alternative replacements for built-in functionality just to reduce their exposure. Convenience wins every time.
Notably, the only other serious competitor in the space is also not open source. Sure, you can probably carefully construct a phone from only FOSS, with some compromises of course. But this is unfeasible for regular users, who have for all practical purposes only two choices. And those same users are unlikely to go for alternative replacements for built-in functionality just to reduce their exposure. Convenience wins every time.