If the President can create Executive Orders -- and those have the full force of the law, why doesn't the President just start making laws? Skip Congress.
The federal government is designed to have a separation of powers. The legislative branch makes the laws, the executive branch carries them out, and the judicial branch plays referee. The whole point of doing so is to impose checks and balances on each branch. No one person can take power without forcing others to give up their power. The struggle then is how much power you can get for your branch, not whether you have it at all, which is a much more dangerous game.
There are examples throughout history of leaders who have gained sole power for themselves. Without any checks or balances on their decisions, they inevitably help themselves and harm the country, which is generally seen as a suboptimal outcome. Such people will promise anything to their followers--until they have the power they want. The three-branch system is supposed to prevent this. So far the system has worked.
An executive order, as originally defined, was to give the president a chance to solve a problem when Congress couldn't act quickly enough or wasn't in session. Mission creep ensued. By 1952, the Supreme Court had to intervene and spell out when the president can and cannot use executive orders. Generally, the closer an order comes to the "express or implied" intent of existing law, that is, the "will of Congress," the more likely it is to be upheld. This is in accordance with the separation of powers doctrine. If the president could make the laws, the executive branch would usurp the legislative branch's duties and make it irrelevant.
Because of politics, the balance between the president and Congress has swayed back and forth. It's always tempting to test the limits of what you can do and what you can get away with. As desirable as it might seem to have your side in complete control, though, history tells us that steady moderation is better than careening from one disaster to another. Good government is boring government.