My guess as to why people write their own forths is:

a. It's easy compared to writing a C complier (or even a half-way feature rich BASIC interpreter.)

b. The easiest way to understand some features of FORTH (like tick and var and colon) is to simply re-implement them.

So if you've already reimplemented half of the language, it's easier to reimplement the rest of it than to internalize how someone else's FORTH works.

I worked on a somewhat large FORTH project in the late 80s. My quip was: "The cool thing about FORTH is that it's so easy to extend the outer interpreter to make a DSL. The bad news is that someone on your project already has."