As a Daz user myself I will say
three four things
1) There are a limited number of Daz assets in the world, it's very hard to have individual assets for each character/thing without some reuse because they cost money. It's just not worth paying for each weapon to be different unless it is a "hero weapon" that is meant to be identified.
2) #1 and #3 are probably the same. #2 has different blade, Suba and pommel, probably different or at least different material skins.
3) Everyone in this universe probably shops at Katanas-R-Us
4) a katanna is a katanna is a katanna. They all look
lame the same in real life anyway