The web sandbox is an absolute delight for users. That users can run javascript code from malicious advertising companies without ill effect is a technological marvel. Consider all the shady nonsense advertising companies do today anyway - and imagine if they had full access to your filesystem when they do it! What a horrifying hellscape that would be.

"Oh, opened our website did you? Don't mind us - we'll just go ahead and install our own 'value add' daemons on your computer that show ads, and exfiltrate your data to improve ad targetting. You can trust us!"