The main reason is that Matt wouldn’t be able to freeload without relicensing WordPress - which would be a massive headache for him and his partners to go through; and the reason would be patently and embarrassingly obvious.
The goal I described earlier is not to make a WordPress clone that just happens to be free of Matt. There’s plenty of low hanging, long ignored gripes and opportunities for improvement. Offer a better, Matt-free product, and you’ll win.