... because it was intended to.
Listen to this Interview with Rasmus Lerdorf, the Father of PHP and be enlightened.
BTW: ITConversations real nice site.
What do we learn from this interview? The main goals of PHP were a easy learning curve, a easy way to solve simple problems. It all evolved through Mr. Lerdorf including more and more fucntionality to suit his users. He puts it this way: "I don't know how to stop it, there was never any intend to write a programming language [...] I have absolutely no idea how to write a programming language, I just kept adding the next logical step on the way".
And the other downside is: PHP works in many ways different than actual programming languages, all intended to make it easier too use for the non-programmer. The problem is once a non-programmer gets into PHP he gets mislead in so many areas, so it will be very hard for him to do some real programming with an actual programming language that has a memory model, where endless loops run endless, where objects are pointers and not copied, where variables have a consistent and thought through scoping, where memory consumption does count, where lifecycles of objects are important and so on and on.