No one claimed, that languages with generics don't exist. But if you follow the discussion about generics in Go, Russ Cox did a thorough discussion of all the proposals on the table and why they would mean giving up some of the core traits of the Go language. As soon anyone suggests an implementation not colliding with the core Go goals, the Go team probably would pick it up quickly.

Or phrased it in another way: with all the years of experience on generics, they still got them wrong when implementing generics for Java, e.g. with the type erasure.