What hope does any other store then have to create a malware free web store if even Google can't? And if they allow installation from anywhere, do you realize that whatever state we are in now, it would be orders of magnitude worse?
If there is some way to get malware into your computer, someone out there will make you do it. That's exactly why installing extensions is so locked down. I don't understand how people think that it will magically all be better if users were given full access to install whatever from wherever. Have you never in your life interacted with an average non-poweruser?