If it was not for "reinventing the wheel" we'd all be coding in cobol or Fortean against IBM supplied mainframes.

In other words - because they can and should (and can afford to and not be hooked up to some third party vendor milking them for the rest of their companies existence).

Why sell your soul to Tableau when you can leverage your large pool of engineering talent to create something that only requires investing once, rather than large recurring license fees forever?

In my opinion Superset actually fills a niche that was open for far too long.

Because it is not rocket science and you need people inside the company that know the insides of it when you want to adapt it?

NIH syndrome?