It's mainly about taste I reckon and the fact that with seafoods it's so easy to get them fresh daily here. Few places in Japan are more than a few hours from the coast. The texture and taste of raw meats is just so different to the cooked variety. If you enjoy the texture and flavour, like I do, then no other reason is needed to explain why you'd like to eat things raw.
I think a lot of the cooking the hell out of meats in western societies arose from there not being access to fresh foods too often. They really needed to cook everything so as not to get disease. In Japan fish caught that morning are on your plate that evening. Awesome!
