> I don't know why, there are tons of smaller companies using it.

Longtime rails dev here. The reason smaller companies are using it is because you can move much faster working on a full stack rails app that gives you SPA-like functionality without needing to incur the performance or operational cost of a dedicated front end.

I've worked for both rails shops and JS shops, and the productivity achieved with Rails is staggering compared to React in a small team environment. Guillermo Rauch tweeted a few months back that SPAs were a zero interest-rate phenomenon and I completely agree. Just because a bunch of companies jumped on the JS hype train doesn't mean that they were all making the right decision.