It is only the job of the executive because Congress told them so via Acts of Congress. Looking at e.g. the firings of the inspector generals, Congress has put very clear language into its laws on why and when those inspector generals can be removed by their post, yet Trump and his cronies ignored this.

It should not be a formality because while it is true that the Republicans have a slight majority in Congress, the founding fathers never intended this most powerful of the three branches to be run by parties. The power in Congress is split up geographically for this very reason, but the party system, that secured its seats with gerrymandering, is highly toxic for a functioning legislative power in the US. It is disappointing to see Republicans in Congress not restricting the executive orders of the new self-proclaimed King.