Now that the subject's come up, I always thought that the "America" civ was more than a little contrived. While the US is certainly a major player on the current glabal stage (for good or ill), we are definetly "johnny come latelies" in terms of global civilzation. History will only tell how significant an impact we'll really have when the dust settles, but compared to the Chinese, Greeks, Egyptians, Indians, Romans, Mongols, Arabs, etc. we're light-weights.
Besides, Lincoln in his little mammoth skin cap is just silly.
And as long as we're wondering why there isn't a particular civ, I've always thought there should be a "Jewish" civ. (And no, I don't mean the hordes of Egyptian slaves . . .)