It's because one day Amazon will be the only place anyone buys anything and thereby it becomes the only company that would need to buy ads, and in a monopsonist

It's because one day Amazon will be the only place anyone buys anything and thereby it becomes the only company that would need to buy ads, and in a monopsonistic auction the winning bid is zero. Seriously. The less competition for ads the less money Google makes. Google has every reason to want a crowded marketplace and not a single 800lb gorilla as its only customer.