This is the answer. On paper, we have laws preventing monopolists from abusing their power. In reality, it's easy to disguise any predatory pricing or "special offers," running at or near a loss in your upstart competitor's market until they go out of business.

The only way you win is either major disruption (which is usually not possible), or having a bankroll comparable in size to the incumbent. But anyone with enough cash to enter the market is doing so for a return-on-investment, with just as much profit motive as the existing players.