I did a little bit of digging on the topic and found this TapTrip blog post: A brief history of Sushi: why do japanese eat raw fish?
It also references a Cultura Bunka article in Portuguese called Uma breve história do sushi.
During Muromachi Period (1336-1573), japaneses [sic] used to transport the raw fish inside of baked rice to keep it conserved during long trips. Then, they started to eat this meal which was called sushi.
But, making sushi was hard because took a long time and was a little expensive. Only during Edo Period (1603-1868) japaneses started to eat the raw fish freshly caught from the ocean with rice thanks to a sushiman called Hanaya Yohei.
So it appears the short answer is indeed "just an accident of culture" (or history) so-to-speak.
I will also quote from a short article Japanese Food Culture of Eating Raw Fish *
Raw fish dishses have been eaten since the Nara-era. At first,
people ate raw fish pickled with vinegar as "Namasu". Then, from the Muromachi-era, people
started to eat "Sashimi".
And a bonus small bit on careful preparation of Sashimi:
Sashimi is the main dish in the Japanese cuisine,
and the cooks consider carefully the best way of
cutting the fish, arranging the fish, shellfish and
squid, give importance to the proper use of
condiments, and the best combination of fish species when serving. The thickness of sashimi is
determined according to the collagen (main protein
in the connective tissue)
content of the fish
* [Foods Food Ingredients J. Jpn., Vol. 212, No.8, 20]
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