I have no idea why anyone would say it's not simple, it's super-simple. Learning how duck typing works with interfaces and how to use it is perhaps the only hurdle. In my experience, only certain BASIC dialects like VisualBasic are simpler.

I think the sticking point is what people mean when they say simple. To me, and likely to many saying Go isn't simple, simple is not a synonym for easy.

Go is easy, but it is not simple. For example, solving the problem of generics in a generic way from the start so the same problem can be addressed in the same way everywhere, would be simple, but maybe not (as) easy. Contrast that to giving the runtime/standard library a special exception with maps and lists. That's easy, but not simple. People used to literally use code generators or weird UTF-8 characters to fake generics, that's not remotely simple.