For example, Congress holds Constitutional authority to appropriate funds to agencies. A great place to see where Congress exercises that authority is via spending bills[1][2]. Depending on the organization, they will get into nitty gritty details to the point of almost micromanaging an organization. As an example of where you do not typically see/look, is the lead up to the creation of spending bills. There is a ton of research, cajoling, persuading, and compromising on all sides of the aisle during that process.
It may seem that there is not a balance of power, but perceptions are notoriously faulty. It is the same way that if I were to watch the news regularly, I would think the world is being taken over by terrorists or that crime is at an all time high - neither of which are true. Things are not nearly as bad as they seem and people have a habit of romanticizing the past.
That being said, there is currently a divided government (e.g., political science speak for Legislative and Executive branches being controlled by different parties). In addition, this administration has had three Supreme Court appointments in a single term. That is pretty much unheard of. The Legislative Branch has also delegated some pretty significant authorities to the Executive, which has been problematic in some cases.
I guess to wrap this up - what you are seeing is not new. The behavior is just more apparent because of modern society's unprecedented access to information and a recent trend of politicians who don not have enough shame to hide their shady behavior.
[1] 2020 Spending Bill - https://www.congress.gov/bill/116th-congress/house-bill/1865...
[2] FY 2021 Intelligence Authorization Act - https://www.congress.gov/bill/116th-congress/senate-bill/390...
The US's problem is that Congress has abdicated most of its power to government agencies in the Executive branch, and politicians are happy with this as it makes them less responsible for failures to properly govern.
This would be the first hypothesis I'd investigate. That politicians would launch investigations for mere political purposes seems plausible.
https://www.libertarianism.org/podcasts/free-thoughts/who-br...
Congress has taken the path of contacting the agency's IG and asking them to investigate and follow up. In a situation where the the IG is a trusted actor, which is usually true, this is a normal and reasonable preliminary step for Congress to take in an investigation.
Congress have the option to get increasingly adversarial in their oversight if it doesn't work out, but the nice thing about working with the IG is, there are no constitutional separation of powers issues with an investigation run by a cabinet-level agency's IG. When the system is functioning halfway normally, bureaucrats and political appointees in the agency must cooperate with the IG, and woe to them if they don't. Contrast that with a Congressional investigation that might be contested legally in a number of ways based on separation of powers concerns. Constitutionally and practically, it's the difference between the executive branch being investigated by the legislative branch and the executive branch being investigated by their own superiors in the executive branch.
The trick is that the system assumes IGs strive to be independent and empowers them accordingly, but the Trump administration has fired [1] multiple IGs that he felt were not friendly to his cause. With the investigation, if the IGs are not working in good faith, the Attorney General thwarts the investigation, the President is a crook, or whatever, the whole system melts down. And in this instance - where Congress is aware that the IG and others might not be working with their normal independence - Congress taking a lighter approach might be deference to the executive branch's authority, but practically, it's recognition of the fact that there's a decent chance there will be new cabinet officials, a new Attorney General, and a new President a few months from now. It's an inappropriate time for them to go nuclear.
[1] https://en.wikipedia.org/wiki/2020_dismissal_of_inspectors_g...