I'm sure if you google it you'll find the official in-universe explanation.
In reality, It's probably just because four eyes and six limbs just looked too alien for the broader audience to be able to relate to. Same basic reason why they're even humanoid at all. A love story would be a hard sell otherwise.
If I had to speculate though, I'd say the Na'vi aren't entirely native to Pandora. Maybe a specially adapted seed species, possibly even part terrestrial hominid. It'd certainly explain the similarities, their relative proximity to Sol and that it's even possible to engineer the Avatars with human DNA at all.