Anti-trust law in the US does not exist to promote competition.

Monopolies are not illegal in the US. The violation of anti-trust law generally occurs when the government 'proves' harm to consumers. It's an almost entirely arbitrary decision on the part of the government as to whether they pursue a company on anti-trust grounds.

Intel maintained a near monopoly in personal computer processors for a long time. They were allowed to keep that position, despite the lack of serious competition beyond AMD's small slice of the market. Cisco, eBay, Facebook, Google and Microsoft have also been allowed to keep their overwhelmingly dominate positions over the years. Why? Because anti-trust has nothing to do with promoting competition.