I'm not sure why you're surprised, Windows has come with a library for certificate validation since the late 90s. The OSX documentation library has an example of using SecureTransport all the way back to 2004, but the API is probably older. The *nix systems, with their modular nature, may be technically usable without a TLS library, but even your average RTOS comes with fully-featured TLS support built in these days.

Mobile operating systems provide a very broad API so that access management and sandboxing is made easy. I'm not sure how things are done on the iOS side, but on Android you can enable certificate pinning application-wide by just putting an XML file with the right name in the right place and adding a key/value pair for the hostname and the pinned public key (anywhere in the validation chain, AFAIK). The same XML file also allows disabling plain text requests from your application runtime, preventing accidental data leaks to insecure networks.

Because adding security is so easy, there's loads of apps enforcing a security setting that otherwise would be considered obscure to most application developers. Exposing an optional, application-wide API is a pretty solid idea in my book; I'm not aware of any Linux system API that can easily enforce certificate pinning on an application-wide level.