PHP isn't an application server. It's tightly coupled to the request/response cycle. There are things you can do to sorta get a longer running environment but for the most part you are loading and tearing down everything for every single request.

Contrast this to Python, Ruby, Node, Go, really any other platform where you tend to model applications with an application server that is long running. In a scenario like this, your application is running all the time and doesn't boot up on every request.

For that matter MS Asp Classic works fine too which is my favorite.

I can only offer my reason: I don't know it.

Why bother eating anything except cold gruel?