It took many decades for power to bleed from the other branches to the Executive. Both parties played a role in this (and everyone liked things like executive orders and signing statements when their side was doing them), but that gets us HERE. Remember Schoolhouse Rock "I'm just a bill"? Outdated.

James Madison wrote of the necessity of the separation of powers:
“The accumulation of all powers, legislative, executive & judiciary, in the same hands, whether of one, a few, or many, whether hereditary, self-appointed, or elected, may justly be pronounced the very definition of tyranny.”