In practice lot of applications hard-coded some elements' color while following the theme for other elements, making dark theme unusable because you end up with black text imposed by the developers over the black background you choose for your theme, and other similar issues.

People can self sabotage by choosing a bad theme, and then they engage with the app less or even churn. Designers need to be careful to not give people rope for them to hang themselves with.

As much as we love to hate on Apple's more user-hostile policies, it's only due to Apple's fiat that we've been able to claw back the smallest bit of user theme control --- light and dark mode --- from the "don't theme my app" people.