Because I like QBASIC better ;)

Self-hosting is very important for the portability & longevity of QB64. QB64 contains many C++ components so it could be argued that any parser written in C++ would do the trick and be just as portable. Still, I'd prefer to keep more of the code BASIC code and less of the code C++ code where possible.

I've never used 'lex','yacc','flex' or 'bison' or any other 3rd party parser before. I know what they are for, but I'm happy to code this for myself. It also gives me greater control over the whole compilation process. Furthermore, I'm not certain that using a 3rd party parser would have saved me a noticeable amount of time when I compare it with the development time I've spent on other areas of QB64.