Besides the fact that I think it's less simple to have ad-hoc compiler extensions for the standard library types that can't be replicated in userspace, so the alternatives are code generation and type erasure, it doesn't have to be an absolute principal! We can have generics without sliding towards some slippery slope of abstraction! You don't allow some kind of generics one day and wake up the next day with custom operators and monads. The buck really can stop at this one pain point.
Okay, that's nice, but the Go developers already picked their side in the debate many years ago. If you don't like it, use one of the many other tools available.