Unless there was a ring of people, each one extracting 5 kilos at home (~50g dmt assuming 1% yield) then pooling everything together and pushing it to wholesale retailers. Something very similar happens with cocaine extraction.

Extracting has been mace very simple, cheap and safe and requires not much knowledge. Synth on the other hand can be tricky, expensive and dangerous and a know-how is required. I can imagine why someone starting a said business would opt for extractions. $100-$200 per gram on the streets (which is not an unrealistic price) can ring the business bells to some minds. Cheap to extract, expensive to sell.