You bring up an interesting topic. I think if you want to understand the US govt you must understand the people who formed it. It was formed by "very liberal people", they were intellectuals and at same time *sort of* "common" people.
I'm not pretending I can explain it in a post- you have to read the history. But I can give a simple view: The founding fathers were intending to build a great nation (it was pretty easy to see that there was this potential even when the US was an infant). They looked at other nations in the past, and asked why these nations failed. Every nation had failed, and all the current nations were hopelessly corrupted- England was probably (objectively) the least corrupted- but "the view" was that this wouldn't last over the test of time. They wanted to at least attempt to build a nation that would survive the test of time and even "save the world".
The main advantage that the US had from their POV was that the US had a "clean slate"- and they could incorporate "lessons" from the past. {the actual fact is the slate wasn't really that clean- e.g. slavery and indians}.
So they had an opportunity build from scratch a "perfect" govt, rather than a patchwork which "evolved over time". You have to understand that the govt was seen as the problem. Power corrupts, absolute power corrupts absolutely. So to solve the problem of power, they build in a separation of power and a balancing of power. For example US president is very limited in his/her power (utterly dependent of Congress for the money to do anything), but in matters of war and foreign policy he is the commander of our miltary forces. This also means that our miltary is controlled by civil authority which is voted into office- a new concept.
Modern presidents have another source of power- the president can talk directly to the public. Therefore through the people the president can influence the Congress. There has "always" been an ongoing debate about the power of the president. I think if the founding fathers were alive today they would be concerned about Presidental power, but the power of Congress would probably be a greater concern.