I think it does largely explain it. Congress can't do even necessary things. On the rare occasions they can, they create an executive branch agency and give it the power to write regulations. They know that Congress is neither qualified nor capable of handling it.

Then when the executive takes more power they have little ability to opposite it. Especially since there is a significant Congressional delegation who believes in the Unitary Executive theory, who will interfere with attempts to rein it in.

I believe it is entirely due to the Founders being intent on pitting Congress against itself, while the President is a single person. They initially tried a half-assed version of that by making the runner up Vice President, but that was quickly seen to be useless.

There is a lot more to be said over a quarter millennium, but that is the fundamental design flaw.