Google basically makes money by:
1) putting all the websites in one place
2) Displaying only the most relevant ones depending on what people search for
3) Sorting the results from (theoretically) the best one to the worst one.
4) Selling ads to business owners who want quicker and more targeted traffic
Now here's an extreme case, just for the sake of the argument.
Let's say all the business owners would have to pay to get traffic.
But only 2 people actually have money to pay.
What would happen is that the rest of the websites would disappear, right? Because people would stop "listing" their website to Google.
If people would stop "listing" their website in Google, it means Google doesn't have what to show.
So, no traffic, no ads for them.
It's a super basic and extreme example, but maybe it helps you enlarge your perspective
+ the reasons all the other guys said above