What accounts for companies like Facebook, Amazon, and Microsoft leaning into RN? Cost cutting and enshittification?

I agree with you that it seems like the only way to get a full-native UX would be to use the platform... but you also probably need to memorize the platform's design guidelines in order to not undermine the framework you're working with. Coupled with the need to maintain 2 codebases (3 if you want web or desktop) seems like enough complexity to warrant specialist developers, making high-quality multi-platform apps categorically impossible for a solo dev or small team.