I saw a good explanation. The President just doesn't have that much power. Most of the power goes to the "Deep State", the unelected bureaucrats, judges, activists, etc.

Example: Trump wanted to crack down on illegal immigrants. Rioters started obstructing ICE violently, well organized violence. Courts and Congress sided with the rioters over ICE. Thus, Trump was forced to scale back his illegal immigrant crackdown.

The President isn't an absolute ruler, like the CEO of a corporation. He has maybe 1% influence on the direction of the government. In one of Trump's businesses, if a subordinate didn't do what Trump wanted, he could just fire them. In government, the disobedient subordinates can just ignore Trump, and then sue and win if Trump tries to hold them accountable. There are more disobedient subordinates than Trump can personally fire.

The election doesn't give the President 100% control of government. It just gives him 1% control, maybe, on a good day.