That would be an accurate metaphor if ads had root privileges on the browser, but web pages are sandboxed - so no, everything is not being run as root.
Ads have root privileges as far as your online fingerprint is concerned.
(not to mention that running untrusted code could expose you to browser exploits so running untrusted code for no benefit is a liability when there are permissively-licensed tools such as uBlock and its filter lists to detect & block them)
