How do they get around copyright? They must have a license of some sort. And those sets cost a good chunk.
Paramount is tolerant of, or at least more or less silent on, fan fiction (including film) provided it isn't made for monetary gain.
Hence, the sets, like the movies, are fan-produced, and since they can't make money off them, a labour of love. Some of these constructions are pretty good - for the Star Trek: The Next Generation episode "Relics" (the one with Scotty), the captain's chair and nav console were rented from the fan who built them. And, of course, modern PCs have enough computing power that it's literally possible to create production-quality video and CGI for next to nothing.