> Its huge advantage is that it's built on the assumption that the user is trusted and the code isn't. In contrast most operating systems are designed on the assumption that code is absolutely trusted, but the user isn't.

It's not necessarily an advantage, it's just a different threat model. An OS is protecting against an attacker already having access to the system (whether physically or over network.) The assumption is that the system is working properly and it's the operator that is malicious.

For the browser, the assumption is that the operator is working properly, but the systems they will be accessing are malicious.

The browser security measures are like a guard at the castle gate, allowing or preventing people from entering. The OS security is like locks on the doors inside the castle so that only people with the right keys can get into various protected rooms.

Both are necessary because they're preventing different things (access to the system vs. access once you're already inside the system.)