1) Mergers are often denied in the US, but once a company is large enough, there's no breakup.
The Bell breakup didn't really help, as some of the Baby Bells later merged into AT&T, though alernate equipment mfgs. and long-distance providers emerged.
2) The US govt. is reluctant to assess serious penalities against large companies because of the layoffs at Arthur Andersen in 2002 due to the Enron scandal.
Also, the SEC will only initiate cases where the trial is affordable and predictable, partly because of the Countrywide scandal where the founder had comparable resources to the US govt., and ended up settling. The case went on from 2008 to 2016.
(The SEC has actually sent letters to large companies asking them to not pay legal fees for executives - pretty desperate.)
https://abcnews.go.com/Business/Decade/arthur-andersen-busin...
https://www.investopedia.com/terms/b/babybells.asp
3) As banks become bigger, they become "too big to fail." Wells Fargo should have been shutdown after admitting millions of counts of fraud against account holders in a systemic effort to increase fees to move their stock price.