The justice was mistaken

"The rise of administrative bodies probably been the most significant legal trend of the last [19th] century and perhaps more values today are affected by their decisions than by those of all the courts, review of administrative decisions apart. They also have begun to have important consequences on personal rights. . . . They have become a veritable fourth branch of the Government, which has deranged our three-branch legal theories as much as the concept of a fourth dimension unsettles our three-dimensional thinking." (Holzer italics.)

Robert H. Jackson, Associate Justice of the Supreme Court of the United States; Chief America prosecutor at the post-World War II Nuremberg War Crimes Trials.

Justice Jackson was mistaken because, as time has shown irrefutably, congressionally-created  independent agencies--possessing combined executive/administrative power (commissioners and staff), judicial power (administrative law judges) and legislative power (rule-making)--do not mimic the separate Articles I (legislative), II (executive) and III (judicial) of the Constitution of the United States of America.

The independent agencies stand outside the Constitutional framework. As such, rather than being a fourth branch of government, independent agencies are a parallel government, with little Article III oversight and thus even less accountability.