This is exactly how I assumed modern operating systems would operate (macOS seems to operate this way because you can actually show size subtallies, but it actually polls and caches; Windows doesn’t even bother unless you get properties on a directory, which is lame; I think beOS may have actually done this?), and was disappointed when I realized it wasn’t… and still doesn’t for some reason? Why not? Couldn’t they just route all OS-level ways of writing to the storage medium through a system like this?
There's no real reason they couldn't but polling systems tend to be the first thing implemented as conceptually they are simpler and require less rigor to enforce. Switching from a polling pattern to an event pattern however is a sizeable amount of work required and you can frequently get bugs in the switch. The discussion here is in the context of a greenfield project where such concerns aren't an issue.
To be clear it's not like an event registration system is better in every way than polling. It's trading some extra hd and (potentially) memory usage for upfront and cheaper cpu costs. It is possible that whoever made the decision weighed the two options and decided on polling instead of events, and this decision was made long ago when os filesystems were first being designed and storage was at much more of a premium.
