I was a PHP developer for over 15 years and those three concepts do more to lower the barrier to entry than any other language I’ve come across, and are constantly underestimated in their power to enable developers to concentrate on the business logic of their program.
Trying to get a python/ruby framework working in the same env, without root access? Whoooo boy. 20 years in the business and I still refuse to do that crap. FastCGI and the like just never work properly the first time in shared hosting.
You need virtual environments to keep dependencies at bay, debugging is a pain if the web server and fcgi process don't communicate properly. Most likely you won't have access to error.log so good luck figuring out why everything returns HTTP 500.
While you're still figuring out how to get helloworld.py to respond, the dude with PHP is already done.
It encompasses every aspect: familiarity, usability, low learning curve, low time to market, enablement, and, eventually, greater margins.
That one is not great, unlike first two points. For one, larger bespoke programms need CI (and would benefit from CD). There is linting/type checking/testing to be done.
Secondly, "copy files deploy" is cheating. Somebody else did the job (hoster) and we are claiming that it is somehow benefit of PHP itself. Well? Did hoster do nothing? Really nothing at all? Nope. On top of that. Performant set up for PHP nowadays do not look that much differently then say, setup for Python.
Two first points are very good. Shared nothing architecture preclude existence of subtle bugs. While multi-threading in mutable, imperative code is the definition of subtle bugs. Those two keep PHP relevant and even on top of it's game (in certain domains).
Same reason as any, familiarity. Not all people, even in software, enjoy learning new stuff.
PHP might be the only language they ever learned and it has paid their bills for over a decade. No point in learning anything new unless it's absolutely required.
As long as major pieces of software are written in PHP, those people will have a steady job.