At Open Knowledge we built a really early one called opendatasearch.org in 2011/2012 - now defunct - and were involved in the first version of the pan EU open data portal. We also had the original https://ckan.net/ (and subsites) which is now https://datahub.io/ and has become much more focused on quality data and data deployment. [Disclosure: I was/am involved in many of these projects]
The challenge, as others have mentioned, is that data quality is very variable and searching for datasets is complicated (think of software as an analogy - searching for good code libraries is a bit of an art).
I imagine Google are trying this out before making datasets another "special type" of search result -- after all you can already search google for datasets. In addition, Google are already Google so including datasets will have a level of comprehensiveness and exposure you struggle with elsewhere (part of the power of monopoly in a sense!).
PS: for those looking for data gov sites https://dataportals.org/ has most of these.