But it being MIT also makes it easy for the company to just work internally by themselves to make improvements for their version of the software without paying anything at all to the original dev. They can even release a better product than the original dev using his own changes and unfairly compete against them without sharing anything back.

Whereas, if the license is GPL, they would need to hire the original dev and colaborate with him fairly if they ever want to make a proprietary version of the software (which can be done, as long as the dev is the sole copyright holder).