(US) Why is the presidential election treated so importantly if the president has the least power out of all positions in government?

Oh, no, he has the most power of any individual in the government - it's just nowhere near as much power as many people seem to think. A lot of his power is negative (the veto) and most of the rest is in controlling who runs each government department, and how; along with appointing judges: so it's mostly indirect power, but it's still unquestionably powerful.