This is mostly true, there are exceptions (ways for apps to provide larger services to the rest of the system like at networking or crypto levels for example and other app services and extensions).
But you are missing the point. The point still remains there is not just one solution, something so fundamental should be part of the OS, otherwise you end up in what crippled Linux, 50 different solutions, not compatible, addressing the same need in different ways, and not maintained and kept on par with OS evolutions, not localized for all markets, not one set of supported APIs and stuff needs to be user installed. So this leads to limited traction / use by third party developers and fragmentation of the UX, leading to complexity or just unavailability of the feature.