Many extensions have full access to all webpages, also in chrome; there's no sandbox or anything similar safely separating extension from page. That's not a bug

Many extensions have full access to all webpages, also in chrome; there's no sandbox or anything similar safely separating extension from page. That's not a bug or missing feature: it's by design! Many extensions fiddle with all kinds of page aspects as part of their core functionality. An adblocker that can't inspect the page dom would likely not work very well.