Because storage is cheap, so it's not worth optimizing that heavily for, because the optimization creates a huge amount of headaches.

There's a reason that today you can just download an app, and it just installs, runs, and uninstalls itself cleanly.

There's no fighting with dependencies, or installing versions of libraries or frameworks before you can install an app, or having apps conflict with other apps, or having bits of app installations lying around conflicting with things.

That's because we used to spend a lot of time and effort making sure that only a single copy of each dependency was installed on a system. If two apps both relied on the same library, one would install it, and the other would then be dependent on it as well and not install its own copy. If the original is removed you have a problem. If it thinks something else is dependent on its asset still and doesn't remove it when it should you've got a problem. If they were both dependent on different major versions of a library, you could run into conflicts and compatibility issues (hello dll hell). Either the apps would have to manage all that, or the OS would, or eventually the user often would.

Now every app just bundles all its dependencies with it. It means the app comes as a clean bundle, there's no conflicts, it can install cleanly, and there's so much less time spend on packaging apps and debugging various system configurations.

Quite frankly this makes way more sense as a model for distributing anything. Yes it costs more in storage, but it pays off massively in resiliency and time savings for everyone.

Also, unless everything is done with vectors, high def image / video assets are not small and can very quickly add up.