Why were bronze swords riveted?

[rupol2000]

cast like the bronze sword in the OP

In general, yes, bronze swords were apparently cast and not forged, and it was probably mass production without a special quality of metal, since bronze still cannot be made comparable to steel, so, indeed, it could just be a saving. The handles could be made from waste.
It could also be due to the different content of tin in the blade and handle.

 

[Unkotare]

In general, a katana is a saber and not a sword, and the Japanese were not there in the days of the samurai, it was most likely the Ainu. So nevermind

What the hell is this moron getting on about this time?