Unix systems were initially designed to be multi-user systems (as in multiple meatbags accessing a mainframe across terminals), in an era before it was common to indiscriminately download and run applications from the internet. Files required explicit opt-in to become executable. There were always attempts at mischief, but it was deemed sufficient to separate user account from each other and denying direct access to the hardware so an account compromise wouldn't escalate to the rest of the system.

Because of this heritage Android uses user accounts instead of namespaces a.k.a. containers (a much newer and less mature concept) to isolate apps from each other.