Uh, because animated mummies, sphinxes, genies and golems are fictional, and samurai are (or were) real?
Ah, so that's the loophole. Samurai Golems, here I come!
I agree that there isn't going to be a great deal of difference between a historical knight and a historical samurai. They're both just human, after all. But D&D has never been a historical recreation game. The classes are based more on folklore and mythology, in my mind. Really warped folklore perhaps (Hello, Druid!) but the kernal is there.
All the same, even legendary samurai just tend to be people who kick arse with swords. Ninja's and Monks get supernatural powers attributed to them, but Samurai are typically just really skilled fighters. This is why creating a good, unique, Samurai class is difficult. Better options for light fighter types would go a long way, too. Swashbuckling samurai heroes tend to be more of the unarmored, speedy, iajutsu types. (Heh, if you want to use anime in a reference, the Samurai mounted on a horse in full armor are just stupid grunts who are destined to die a quick, gruesome death)