Let's take TV as a comparison. On TV the ads have guidelines on what they can do and show, set by the tv channel and the broader regulation.
On the internet, there isn't anything, it's the wild west. It can't hurt to get some minimal standards, doesn't matter where it comes from as long as it's enforced.
Online advertising has existed for over 25 years. [1] Online ad brokers (i.e., competitors with Google) have existed for nearly 20 years. It is incredibly disingenuous of you to pretend as if Google is in any way "forced" to play both gatekeeper and ad broker, like no-one else is trying to compete in that space. Developing Chrome was a deliberate choice by Google to position themselves to own the means by which users consume ads, search (ads), and video (ads).
Legally, it matters a lot whether it's the government enforcing rules which restrain trade or a private coalition including the major incumbent players in the industry being restrained.
On the internet, there isn't anything, it's the wild west. It can't hurt to get some minimal standards, doesn't matter where it comes from as long as it's enforced.