Why does what keep happening? Antitrust cases or companies becoming monopolies?

Because it's very profitable?

The issue is that even if you know it's wrong, what are you going to do to. You grow to a certain size, and it then becomes clear that you're inching closer and closer to becoming a monopoly. Do you then stop and inform your shareholders that you unfortunately reached the maximum size for your organisation?

Even if you have no moral issue continuing down your current path, you have very little to fear, because the precedence is that you'll get a relatively mild punishment.

For Amazon, if this was to have any real meaning, the US government could step and an say: Because of the actions of Amazon, the online store, we're no longer able to utilize AWS in good faith. This would force Amazon to split their businesses into two, or lose government contracts, which would both be a real punishment.

Monopolies are a feature of unregulated capitalism. Every company wants to dominate the sector they're in, competition invariably leads to monopolies.