"We" are a minority.
The vast majority of websites out there are still hosted on shared servers over which the customer has very little control other than installing and updating WordPress. You might be able to convince cPanel to let you run a Rails app, but the system as a whole was designed with PHP in mind.
The owners of those websites have absolutely zero interest in messing with the server environment. They're not developers. They've probably never heard of devops. They're totally allergic to SSH and can barely even use FTP. And yet, these are the people who, whether knowingly or not, help keep the web as decentralized as it is. PHP was the only language that catered to them 15 years ago, and it still is the only language that makes this possible on a large scale.
I host my blog with NearlyFreeSpeech.net. The whole site is just a couple of PHP scripts. I haven't updated it in almost a year, but it keeps working, no maintenance required. If I had to host it on DigitalOcean like all the cool kids do, it would cost me 10x more in monthly fees (even with the smallest droplet) and 100x more in time. This kind of setup is what I call "It just works." Not dockerfiles and bootstrapping commands and all sorts of dependency management tools.