Why do we allow megacorporations to exist at all?

Chicago School assholes backed by the conservative/"pro-business" think tanks that boomed in the post-war era managed to place some judges and influence some lawmakers to completely break our anti-trust enforcement in the '70s, by shifting from a standard of "enormous corporations may be assumed harmful" to "specific harm that's very expensive to prove must be demonstrated", all but entirely eliminating anti-trust action.

The term "allow" implies that some entity called "we" exists, and has the capacity to exercise control over over unrelated third-parties' ability to engage in organized activity. No such entity exists, nor has such capacity.

Google “Capitalism”