> It's why I don't understand why in the US, the president is also the top military commander.
Simple: the founders of the US thought it would protect our democracy to ensure that a elected civilian was in charge of the military. It's mostly worked.
Isn't there an even simpler answer? Back then most countries had their king ad their top military commander and the president of the US had a very similar role to a king.
Nope. There's a lot of writing about this issue from various founding fathers. There was great desire to avoid the President appearing to be a king or turning into one.
But it was still the model they were working from, the way of organizing state power that they understood and were familiar with. The three branches gov is in a lot of ways a secular and formalized form of the constraints religious and aristocratic political influence had on european monarchy in the era just preceding the revolution.
Plus what they say they wanted to achieve is not necessarily what they did achieve. That's not to say they failed and the president is a king, but the president is not entirely un-king-like either.
Simple: the founders of the US thought it would protect our democracy to ensure that a elected civilian was in charge of the military. It's mostly worked.