There's no law against monopolies.
There are laws that prohibit becoming a monopoly by merging with your competition. That's why mergers have to be approved by the government (many governments in the case of multinational corporations). Often mergers require spinning off divisions or other conditions to maintain some level of competition. Some have complained that regulators have been too lax or have applied the wrong standards in approving deals that lessened competition, but the point is that we do have laws. Amazon in acquired a lot of smaller companies, but most of their dominance has been grown internally.
There are laws that prohibit companies from abusing their dominance to force out competitors or to use their monopoly in one market to force a monopoly in another. These are the rules that led to the AT&T breakup and almost lead to a Microsoft breakup. In retrospect, many people thing the AT&T breakup was the best thing that could have happened to the company, and I would assert that Microsoft would have been a much better off if it had broken up into several separate companies. These are the rules that Google is often accused of breaking, using their dominance in search to gain dominance in other areas. All large companies have to watch out for these rules.
But if a company becomes a monopoly without buying out their competition and doesn't use their position in such a way to block potential competitors, then they are doing nothing illegal.
In short, there are no laws against monopolies in general, only against abusive monopolies.