If you're Google with a strong culture of web performance and have generally solved all the normal server issues with a globally-distributed network of edge servers, a 24x7 team of very talented engineers continuously monitoring and optimizing, etc. you might quite reasonably conclude that the biggest remaining area to improve page performance is chipping away at the long tail of users on high-latency connections, unreliable / flaky ISPs which might drop or delay requests for your render-blocking resources, etc.
The trick is realizing that relatively few people work at Google and most other places have not made the same investments to get to the point where this kind of micro-optimization is a net win versus the maintenance costs. Most sites tend to show easier problems - optimizing DNS, getting a CDN or using one better, reducing total transfer size, etc. – which are going to impact the user experience more than inlining some CSS.
This is particularly true as we rapidly head into an area where things like HTTP/2 are changing most of our optimization weights – e.g. packing things into CSS bundles is rapidly becoming an anti-pattern since it reduces the period where a cached resource is still valid.