Walmart competes with Mom and Pop grocery stores. Google is competing with god damn Amazon. And possibly future startups that might otherwise try to enter the smart speaker space, but decide not to because they can't compete at that price point. In which case, great...that is not a case of a large corporation leveraging their size/position to harm customers.

(I work at Amazon)

I think you could argue Walmart was primarily competing with mom and pop stores a decade ago (though really other large supermarkets at that point), but they are most certainly competing with amazon today.

How can preventing competition be great? If innovative startups are prevented from entering the market because of predatory pricing, it does harm customers.

Because human attention is scarce, it's infeasible for them to care until there is a real risk of such behavior becoming a problem.

People would suddenly start to care if the entity enjoys a large enough market share to make them a monopoly.