A very short answer would be: because the Founding Guys didn't want tyrants.

They knew enough Roman history to be aware of the dangers: that a successful general could use the legions to install himself as emperor ( like Julius Caesar), or the legions themselves could un-install an emperor and put another in his place ( like replacing Caligula with Claudius). And there was very much an opportunity for this to happen: war was the business of the King, and, in the colonies, the royal governors. Once that royal authority had been removed, what could take its place to control a national army ? It was obvious that a national army was going to be needed. This was a very sticky problem, and when the Constitutional Convention met in 1787 to work out a new national government there was a great deal of concern about it, and they ended up dividing "war powers" between the Congress and the President. Constitutionally, only Congress has the power to declare war, the President then has the power to wage war.

Now, that turned out to be too simple. On one hand, for the President there have been many conflicts that were not existential threats to the US that still needed an armed response. On the other hand, for the Congress the powers of the President become so great once a war is declared, that it is extremely leery of declaring one. So , the last time Congress actually declared war was in 1941. Still, though much, much more could be said about this, the "war powers" are still implicitly divided. Congress intervened when it was concerned the President was overstepping his authority with the War Powers Act of 1973.

But even before the Constitution had been written there were concerns about the dangers of generals and standing armies. Standing professional armies had been imposed upon the colonies after their militias' rather mediocre response to the French and Indian War, and paying for them became one of the grievances causing the 1775 revolt. The officers of the Continental Army formed the Society of Cincinnati in 1783, shortly after the Treaty of Versailles ended the War for American Independence. George Washington also resigned his commission in the Continental Army in December of 1783, and was called "the American Cincinnatus". The point was obvious: Cincinnatus was the famous general of the Roman Republic who left plowing his fields to lead the Roman army to victory, then returned to plowing his fields. In retrospect, it really seems a good choice by Washington. His ambitions were typical for the Virginia elite, to be a wealthy planter, and he very much wanted to develop his extensive landholdings. But after the Battle of Yorktown and before December 1783 Washington was the most powerful- and the most generally popular- person in the United States, and could have possibly, just possibly, become a dictator if he'd wanted the job.

Regardless of Washington's real motives, that precedent has been very important. Hanging in the Capitol Rotunda is a [very big 1826 painting by John Trumbull](https://www.aoc.gov/explore-capitol-campus/art/general-george-washington-resigning-his-commission of George Washington resigning his commission). Senators, congressmen, their staff, tourists- all pass in front of it. It's like a very big sign saying, generals don't become dictators here.

This is not to say that generals have not been political. Other than Washington, generals have successfully run for President, like William Henry Harrison and Dwight Eisenhower, and many times being a war veteran has been crucial to election- as for most of the Presidents of the later 19th c. and several following WWII . And there have been some very confident generals- Douglas MacArthur possessed enough ego perhaps to consider becoming an emperor ....But Washington's precedent is a very old one, now.