Let's say 5 companies in 5 different markets did break antitrust laws and the government focuses only on company 1's actions and doesn't even bother looking at what 2-5 do.

In that scenario, I'd say 6 parties are acting improperly: all 5 companies plus the government.

That is the unfortunate reality of how most of civilized society works. E.g. sensational crimes get more attention from law enforcement. Laws are applied according to societies whim. I think the natural conclusion of the “what about X” line of argument would lead to the paralysis of the legal system.

I don't see that as a particularly "civilized" part of our society. More like pitchforks and torches than good governance.

Except those five companies are dwarfed in size and market power by the one under spotlight.

The biggest chunk of US S&P 500 market cap is FAANG stocks.