It's not popular because it's a fad, it's not about replacing good old static websites with fancy over-engineered JS code.
It's about making desktop-class applications more accessible via the web. Desktop-class Apps have lower latency requirements then server rendered frameworks are capable of delivering, plain and simple. You could certainly build Facebook as a fully server-rendered PHP app, but that would hurt Facebook's business because its servers would need to do more work and its users would have to wait longer for content.
Fully server-rendered frameworks are not capable of delivering low-latency desktop-class applications. If your app doesn't require low-latency updates, then you can certainly use a classic PHP or Ruby on Rails stack with no problem.
Sure, you can use JQuery style code to make your PHP app more interactive, but you're probably just going to end up with a messy, hard to understand JS code base eventually if you you don't have some sort of low-latency client-side declarative templating framework.
There's certainly some companies that are caught in the hype and build a SPA app when they would probably be better served with a simple PHP site, but on every project I've worked on that used React across a few companies (interactive apps for ex: cropping & manipulating images, building visualizations and reporting on data), not using an SPA framework would have slowed down development dramatically, or left us with a really poor product.