Because they do not share your belief that Go has "fundamental issues and well understood design issues".
If they did then your argument reductio ad absurdum would make sense. However, they don't; it does not.
I think it's safe to say they are aware of the issue and consider lack of generics a negative (because it slows you down, duplicates code, adds type unsafety, etc.). They don't have a solution they like yet.
(1) them not believing that Go has design issues
for my argument to not make sense. It would need both (1) and
(2) them being right about said belief
for my argument to be invalid. And while (1) might hold, (2) is highly debatable.