Because a least one person has to have forwarded ports for them to form a direct connection. [0]
This will degrade torrent performance and make torrenting worse, routers normally have uPnP enabled these days so we forget about it, but this will make it so you can’t connect to any other users who are also using Mullvad, for one.
From what I understand, uPnP took off for a while, but started to become much less common about a decade ago because of the security issues it caused. I think most routers come with it disabled by default now. (If you know of any surveys indicating otherwise, I'd be curious to read them.)
Part of it is that hole punching became a standard feature for new protocols, so the need to forward ports has been reduced.
This will degrade torrent performance and make torrenting worse, routers normally have uPnP enabled these days so we forget about it, but this will make it so you can’t connect to any other users who are also using Mullvad, for one.
[0]https://superuser.com/questions/1053414/how-does-port-forwar...