Because the source is sending SYN packet to the Destination, and wait for a response. If the port was open, the goal was to send a SYN/ACK back to port was the target, but if it is off, it will send back RST.
The source sends a SYN packet to Destination and wait for a response. If the port is open, the goal was to send a SYN / ACK back, but if the port is closed. The goal was to send a RST back.
Because source will send SYN packet to Destination and response. If the port is open, the target will send SYN / ACK back. But if the port is closed. The target, it will send the RST back.