Actually, most operating systems were designed as multi-user systems where users cannot access each other's data. Trusted administrators installed software. This made sense in a world of expensive computers that needed to be shared.

In the modern world we do not share our computers with other users, and allow untrusted software to run on our machines from the internet. Without further sandboxing, it would execute as the user with access to everything for that user - which in practice means mostly everything.