Having worked for one of those company's, I think the productivity of Rails is overstated. Yes it's quick to get a rough demo and POC working (which is already plenty quick in many other frameworks) but once you get into the weeds of actually building and iterating the difference is not so great. I think particularly if you look at these established companies, what tech stack they are running often is just a matter of what the founders knew at the time. The business wasn't successful because they chose Rails, it was a successful business who happened to pick Rails to start with.