Maybe the useful content is something you don't know is there, so you settle for what's in the snippet. Because you imagine Google's AI surely extracted the right bits.
There's also a sort of diminishing returns effect here. If google trains people that the snippet is good enough, less traffic goes to the site. Eventually, enough to shutter the site, for some sites. Then nobody has the info.
The pattern has already affected Google referral traffic to Wikipedia. Pageviews for Wikipedia are roughly flat from 2012 to today, where they had marked growth prior. 2012 is when Google starting rolling out their knowledge graph that presented Wikipedia data directly.
