Because the wind is not pushing against a sail that is perpendicular to it (with the wind coming from behind the boat) – if it were you would only be able to go as fast as the wind. Instead, the sail is angled (e.g. at say 30 degrees off perpendicular, with the wind coming from the side of the boat) so that you effectively create a horizontal and vertical component to the push which adds up to a speed greater than the wind itself
[Here](https://youtu.be/u5InZ6iknUM?t=382) is an explanation that made it click for me
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