> You probably never noticed this, but physically, an Atari joystick port is just a serial port with a lightly modified design

It's not any kind of serial port, modified or otherwise. What makes a port "serial" or "parallel" is the way the data is transferred, not the shape of the connector.

Each joystick signal (up, down, left, right, trigger, paddle A, paddle B, plus power and ground) has its own pin. That makes it a parallel port.

Now, the Atari SIO port, mentioned by another poster, and used to connnect peripherals (such as disk drives and printers) is a serial bus, but the joystick port is parallel.