Because.
Were means "Man" and wolf means...wolf. The words together more or less translate into "man who turns into wolf". I don't see Argonians or Khajiits as men, but beast--if they turn into Werewolves they're a beast that turns into another beast (i.e. you don't see birds turning into other types of "beasts"); a wolf. Also, in nearly every IP that has Werewolves as part of their lore its a man (Human) that transforms into the animal. I guess you could say it's a notion that I've gained, and will continue to keep, from nearly every IP with Werewolf lore.
I just can't get past how silly an Argonian or Khajiit Werewolf would be.
You do not turn into a half-you, half-wolf when you're a werewolf. You turn into a werewolf, a bloodthirsty monster that
resembles but is not actually a cross between a man and a wolf. You are Hircine's [censored] now, and you will turn into what he tells you to turn into whether you like it or not. It's a divine curse spread by a disease.
Also, Argonians seem to be more tree than lizard given their bizarre relationship with the Hist (plus they also have mammal and fish characteristics) and Khajiit are descended from wood elves.