To paraphrase Donald Rumsfeld, you get the Constitution that was given you, not the one you might have liked.
The U.S. Constitution has many virtues and is a remarkable document. The holding of the legislative and presidential elections are fixed in time, and this makes for great institutional stability.
But the perspective of the times that the Founding Fathers had counted heavily. They did not want an overpowering executive which they had been subjected to by the British Crown. They had had no representation in the British Parliament. (Hence the slogan "no taxation without representation").
As the American Constitution was put together, the legislative branch (House of Representatives and Senate) was the lead organ of government. It is the subject of the lengthy Article I (The executive is the subject of Article II and the judiciary Article III).
Separation of powers means separation of government. There is no institutional connection between the legislative branch and the presidency, unlike in other systems. The president does not have the power to dismiss the legislature and call for new elections in the event of an impasse, unlike in other systems. The House of Representatives has the power of the purse; the presidency does not.
With the unhindered practice of gerrymandering (manipulation of the boundaries of House constituencies) so that a significant number of seats are in effect one-party seats indefinitely, a determined faction in the House can constitute a force that potentially can block the funding of the government. Such is the crisis we have come to today. It brings government and governance to its knees, citizens suffer mightily, and there is no recourse.