They're not always sitting idle waiting for one big OLAP query (well suited to parallel aggregation) to come in to exhibit a 25x speedup.

Let's say you've got a web application with some cookie-cutter OLTP queries (which are well suited to serial plans ... no parallel aggregation benefit here). Your CPUs are running at 50% then some big OLAP query is going to come in and get maybe ... 13x faster.

Which is a nice performance improvement, but not 25x.

These numbers are a good validation of the work done in PostgreSQL, but you can't look at it and look at your X core machine, and say "my workload is going to be 25x faster!"