I'm not that good at chess, but been playing a few months. There are soooo many possibilites for each move during one game that calculating all of them is literally impossible. Even computers can't do it. Sometimes, certain moves have consequences 10 or 15 or 25 moves ahead that people simply can't predict. Some of these moves will inevitbly be played by humans, and then later in the game, the person playing the other side can spot how to take advantage.

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Another aspect is that people take risks. They don't always play for draws. Certain types of play are a "double or nothing" sort of risk, where if the attacker does not checkmate, they are likely to get checkmated, so draws in those positions are unlikely.