Straight Swords for Samurai?

[ItMe]

Samurai use katanas, katanas have a more-or-less-fixed shape, the game is made in Japan.

Go figure the math.

Want a straight blade, become a Paladin.

Not necessarily.
Media may present one shape to the katana, but there are straight blade katanas. The curve is actually something that was added over time, as the Chinese sword the katana evolved from had a straight blade. This does mean that straight blade katana are generally much older, and thus rarer, but they most definitely existed and not just as an oddity.

Either way, the way Ronkans or Allagans make their katana doesn't need to mirror the way they've been made IRL. I agree with OP. Some variety would be good.