He didn't do it because he had to but because he wanted to
On my day job I do things that I have to and get paid for. Here, on a contrary, I have a choice

But, being serious, using external php interpreter is not trivial thing, and suffer from several drawbacks:
1. External dependancy
2. Installation complexity
3. Dependancy on php version currently installed
4. Dependancy on configuration of installed php
5. Developing such thing would be plain PITA - I did some RTFM to be sure
Implementing own interpreter (parser is only one part of it

), on the other hand, is fun and serve the purpose as I see it.
Worth note, that it doesn't prevent me from using external php as choice, and I may add support for this in near future.