Can someone explain to me why a serological study with a large, random sample has apparently not been done anywhere yet? I don't understand why it hasn't been done a long time ago, but there has to be a reason, so I would like to know what it is.

Apparently, one explanation may be that it's not that easy to develop that kind of tests, though I still find it surprising China, where the pandemic started back in December, still hasn't done it. https://twitter.com/jdl288/status/1246847903987126275

If you are knowledgeable about this, it would be great if you could explain the kind of challenges involved and/or point to resources that do. I'm also curious about when/where the efforts to develop this kind of tests started.