But so much of macOS is also uninstrumentable except via undocumented APIs that are unreliable at best. Why wouldn't Linux or even FreeBSD be even better? They're not limited to content but even the source that can be updated and rebuilt to do literally anything.

Honestly, as a long time Emacs user, agents are amazing because when all your functionality and data lives in open source code and text files, the world is literally your oyster.

Linux and the BSDs might be more instrumentable, but the rest of the experience is so fragmented in comparison. Regardless of category, there is no single service or data format that everybody has agreed to use and build on top of.

The advantage of the Apple approach is that the user’s data is instantly available across third party apps. For example, instead of needing to build (or import) a whole CalDAV apparatus to be able to build a little calendar-based todo app, you can just use the system calendar API and let the OS do the heavy lifting, making your app much more lightweight and instantly usable with almost no user configuration. This naturally applies for scripts and agent skills, too.