Not all Japanese movies are anything like that! Grave of the Fireflies has a specific purpose and the bleak hopelessness is required for that story. Watch "Nausicaa of the Valley of the Winds" sometime. Or "Kiki's Delivery Service" or "Laputa" or anything else Studio Ghibli puts out.
I'm no cultural relativist but modern Japanese culture is no more hopeless or depressing than modern American culture. Just weirder.
Of course, not all Japanese movies are about hopelessness, but the cultural situation for Japanese and Chinese is rather bleak like German culture.
Their drama has at its base a powerful sense of futility, and the proposed solution is that one simply endures and suffers because death is thought to be something worse.
It is only in strongly Christian societies that hope is the foundation of human life. Even Jewish life, while not hopeless, tends to see life as more trial than joy. All other religions end in fatalism.
Anything from Miyazaki's studio is terrific. You mentioned a couple of my favorites, to which I'd add "Spirited Away," "Pom Poco," "Whispers of the Heart," and several others.