Google hasn’t just monopolised the ad industry, but is holding the internet itself in its clutches.
They own and control one of the biggest operating systems on the market, giving them control over peoples devices. They own and control the biggest browser engine on the market, giving them control over the internet standards; they can implement whatever the heck they want and essentially force it as a web standard. They own and control the biggest search engine on the market, giving them control over what information people can see and access.
For a lot of people, Google controls practically the entire internet access chain, and that should terrify anyone who cares about a free and open internet.