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.