Think about it let like this,

When you open a port on your router you’re allowing inbound connections from the internet to that port via your WAN IP, with a VPN however, that typically does not work since you’re forcing all your traffic through a specific data centre with its own firewall. Most well developed VPN providers will offer Port Forwarding as a feature which usually opens a random port for people to make those inbound connections, you instruct your download client to use the port your VPN provider gives you so that people can leech from you without exposing your WAN IP.

Hope that makes it easier to understand.