PHP still constitutes ~80% of server side programming languages and a whopping ~40% of the web. So its doing pretty well.

And yes, your argument has merit - the 'modern' frameworks and libraries like Angular, React cater to very specific needs - sites/apps that need to send frequent updates to the user/visitor, sites/apps that have considerable moving parts and therefore need a powerful, interactive front end capability, and sites/apps which dont have an in built templating system. They also provide a base to organize your front end if the framework that you are using does not already provide such organization.

Stock trading services, social networks, betting sites - all of those would require newer frameworks and libraries - they fulfill all needs. So no wonder Facebook is behind React.

But the majority of the internet and web, from ecommerce websites to blogs to other apps, dont need any of that functionality. You dont want to distract a shopper from the checkout process. You dont have many moving parts in a blog. So majority of the web goes without these technologies, serving fast, server-side rendered pages to visitors.

Its a matter of requirements, really.