Getting away with pirating forum software?

There seem to be lots of websites where people can just download leaked/nulled/pirated versions of forums software. But not just forums software, themes, add-ons, plugins and much more for them. I was wondering if anyone could expand more on this.

So how do so many people get away with using this pirated forum software andwhat would happen if someone was caught using it?
Who would be enforcing this, and how would they find out?