That’s because treason has a very specific definition under the Constitution, to wit;
Treason is a very difficult crime to charge and prove. Even the Rosenbergs, some of the most famous traitors of the 20th century, were only charged and found guilty of espionage. Almost nobody has even been charged with treason, let alone convicted, since the end of WWII.
You need to be a lot more specific than that. Congress has the authority to approve or reject the president’s nominee for Attorney General and certain other DOJ officials, and they may create laws that they expect the DOJ to uphold, but they do not have the authority to tell the AG what to do. A criminal referral from Congress is a recommendation, not an order.
Congress has certain powers over the DOJ, as with any executive agency. Budget, making certain positions subject to Senate confirmation, changing the laws they enforce, etc.
But, the head of the executive branch has the authority to direct the agency’s policies, or give orders to enforce or not enforce. Congress can’t impinge on those powers.
Congress could pass a law saying that a certain person is guilty of a crime, but that would not be enforceable because it’s unconstitutional.
Likewise, passing a new law defining what the targeted person clearly did do as a crime, and applying it retroactively would be unconstitutional.
The DoJ may be ethically obligated if the evidence is strong, but they are not legally obligated to follow up on criminal referrals. That is necessary due to the principles of prosecutorial discretion and separation of powers. (Prosecutorial discretion is a strictly executive-branch power in our federal government.)
When Jane Fonda was arrested on drug smuggling charges in the '70s, it was said to be on Nixon’s orders, not Congress’s.
For a real life example, when members of Congress made a criminal referral regarding Chris Steele (author of the Steel dossier), it was reported as a “request”.
When Congress wants to direct an investigation, rather than simply recommend one, they must either appoint a special counsel (formerly independent counsel, examples include Mueller and Start) or perform the investigation themselves. Under no circumstance may Congress direct a criminal prosecution. Even with special counsel, Congress may only compel the production of a report; the decision to prosecute and the right to represent the government in a courtroom is strictly executive, and in that respect Congress is powerless.
However note that, at least in the past, the DoJ’s internal policy was to not investigate matters occuring before Congress unless Congress referred the matter. (This was something I believe former FBI director James Comey testified about years ago, regarding perjury before Congress as I recall.)