It's a bit amusing to say "see, look at Japan! It's homogenous and it's working well!" Only the answer is "not really." It's hard to advance in Japanese society if you're not Japanese; foreign brands like Samsung tend to struggle (it'll actually hide its name when marketing Galaxy phones); and the country isn't nearly as influential as it was in the '80s and '90s. There are also deep-seated cultural problems, such as overworking habits that make Americans seem relaxed.
As for why you need diversity: for one thing, there are cultural and practical issues a homogenous group may not consider when designing a product.
Take Uber as an example. Even if you discount the harassment problems created by its male-dominated culture, it also led to functional problems with its app: the company hadn't really considered tools to help prevent and report sexual assault and harassment from drivers because, of course, those are far less common problems for men. Why would anyone be worried about the mere act of hopping into an Uber car by themselves? It wasn't until incidents that the company improved its code, but it probably would have been aware of those problems very quickly if there were more women in key positions in the design team.