Honestly? Because PHP sucks. Part of it is tribal loyalty - not wanting to be seen as the sort of person who would write PHP; I suspect more people know it than admit as much here. But mostly it really is a really bad language.
I am a php developer for years and truthfully, I don't find it sucky
I'm an unashamed PHP expert. There's a ton of money to make in fixing other people's PHP, and it's often easy work.
That doesn't mean PHP is "good". With discipline, you can write readable, DRY, easy-to-maintain PHP. That doesn't mean the same code wouldn't be more readable and easier to maintain in another language. While PHP has great documentation and is "easy" to write, it has lots of surprises and inconsistencies.
Although IDEs with autocomplete, static analysis, and XDebug support have made PHP much better, its typing is still a problem. When you're writing extremely large systems, you end up with some very sneaky bugs due to PHP's weak typing.
Google "PHP sucks" if you want to read more.
(Note that Python, Ruby, and JavaScript also suck. I'm really excited for the Go/Dart/Scala/Rust/others era where we've built languages that fix the mistakes we discovered while writing huge applications in the last few decades.)