You have a point but shelf space is physically limited. Online real estate is not so limited. In my country there is reasonably healthy competition between supermarkets. Supermarkets do have self-branded products but they don't cross-sell competitors self-branded products.
Here we have Apple with Google and Bing on their shelves. Microsoft have Bing and Google on their shelves. And Google have Goggle or Bing. Is that healthy or an oligopoly?
There's a reason web designers call specific pages "valuable real estate".
For example Google's search page, the one with the input, is probably the most valuable web real estate in the world, closely followed by the first page of results once you've typed your query and hit Enter.
I'm willing to bet $100 that the second page of results probably gets less than 1000th the hits the first one gets. Heck, make that 1 millionth of the hits the first results page gets.
That's silly. Everything is limited by the scarcity of human attention spans, not just websites.
Shelf space refers to the market with which someone competes, not whether people are thinking of a candy bar or finding a bathroom or a facebook post. Your argument commits survivor bias because it's ignoring the millions of other websites that exist and are being used. Being popular does not mean something is a monopoly, nor does it mean there's limited shelf space.
Following your example, if Google spammed Pixel ads on it's home page, the page would become less popular. One of the reasons it became so popular was it's strict adherence to focusing on utility.
Even if true, what does that have to do with shelf space? Search results do not constitute the internet, and there are many more search engines than just Google.
Here we have Apple with Google and Bing on their shelves. Microsoft have Bing and Google on their shelves. And Google have Goggle or Bing. Is that healthy or an oligopoly?