Moore's Law. Double your workload every 11 months.
Moore's case here is different. It's not reinventing the wheel. Their product is/was Forth, so it was a technical / strategic decision not to rely on 3rd party code. Their core competency is the language and the compiler and how well the code runs, so it's in their best interests to rewrite that particular wheel in a platform-specific, controlled way.
As others pointed out, Moore's decision to rewrite Forth for every platform might not have been the best use of his time. While he personally may have benefited, it would have driven PHB's insane.
If I'm just making a vanilla website that runs on any old server, I don't start by writing a web server, I use a platform. If I want graphical buttons I don't start with rasterizing routines, I start with the GDI routines or whatever is provided. I only step outside and do it myself when I've determined what's out there is junk.
Maybe I need a web server on small devices. I'm writing the web server because I can't or won't use IIS.
Reinventing wheels does come at a cost. Library code has been debugged, and usually if there's an issue, it's because you're doing something wrong and not the library. If you create the code yourself, you give yourself more points of failure, more code to maintain. You don't want to go to the extreme of doing everything yourself because you otherwise spend all your time redoing what's already done instead of making progress.