I imagine it's because the it's "host"ing the program in question. But yeah, without port forwarding of some sort, there's no way your PC can be a host (STUN or whatever doesn't count, because that is the host).*The term "host" dates back to the 1960s and thus to the era of mainframes. Don't ask me why the term "host" is used instead of "server". If I understand the article above correctly that mentions both terms, it seems that "server" is usually used to refer to the program and "host" refers to the machine. But I have very limited half-knowledge and am happy to be educated if I am wrong.
There's a relevant thread all about it: https://forums.frontier.co.uk/threa...imizing-multiplayer-connection-issues.583861/It's not a "last resort" mechanism - port forwarding is an easy workaround for problems that are difficult to solve without actually coming to your house. It's on par with "turn it off and on again" as far as "solutions" go, i.e. a shortcut that gets the laymen to stop bothering you with problems they don't understand enough for you to walk them through remotely. It does work, I'm not arguing that - but it is not required, and is not the recommended approach.