You don't just need the detector though, you need the landowner's permission and the time. And the already tiny chance of finding anything of non-trivial value

You don't just need the detector though, you need the landowner's permission and the time. And the already tiny chance of finding anything of non-trivial value gets smaller if everybody else is doing it. If you think we have a lot of people finding ancient gold coins, you'll be even more amazed how many lottery winners we have!