Routing is a feature because you can express more complex semantics. Most php apps depend on Apache specific .htaccess files to handle redirects and other routing, and it becomes a mess quickly.

PHP files can be swapped out live because many CGI wrappers don't cache anything in memory, which means at high strain you start dealing with disk load, file locks, etc.

There are many other languages which also don't require compilation, often at the trade off of less performance. Some really clever languages let compilation be an optional step.

A single layer means your frontend logic, data models, and database access all run the risk of being one giant mess. Sure other languages run that risk too, but it's especially prevalent in PHP.

PHP can extend itself - it can also be used to download attack vectors and execute them directly on the server with no concept of code signing. Other languages offer powerful facilities for remote updates if they're desired, but that's usually not the best approach for Web.

You can edit other languages with notepad too.

Being able to deploy a single endpoint to a live server and hoping it doesn't cause problems sounds like terror.