Because the tools we currently use (mostly programming languages) are too insecure to allow you to run any random code from the internet (that has full access t

Because the tools we currently use (mostly programming languages) are too insecure to allow you to run any random code from the internet (that has full access to all your computer resources). And most users would be incapable of keeping proper security practices needed in this case. Installed software is considered to have a distributor, ie. a legally responsible entity that can be punished for any shenanigans. I believe this is an outdated model and that with better tooling we could do what you envision.