I've read online that SYN flooding is when "the attacker sends repeated SYN packets to every port on the targeted server." However, HTTPS only runs on port 443. Therefore, why is it productive for an attacker to target "every port" if only port 443 is responsible for web traffic? Why does a web server care what happens on ports <443 and >443?
As a related question, to mitigate SYN flood attacks, why doesn't the server just stop listening at every port except 443? That way, the server can continue on with its business and not have to deal with incoming SYN packets at other ports?