It’s not normal for Japanese people to shake hands, they have a custom of bowing when meeting people. I imagine this means they haven’t developed the practice of shaking hands and maybe don’t understand that a firm grasp has meaning.
Also, many Japanese are timid and maybe don’t want to shake hands but try anyway. But you will find some, maybe who work in international business, who do have a firm handshake.
That’s not to say you won’t run into a Japanese person who does a have a firm grip though. They do exist!
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