Because, they take their height from the Na'vi DNA. An avatar is a mixture of Na'vi and human DNA. Consequently, avatars look like Na'vi, but have some differences. Namely, 5 fingers, instead of 4; the human DNA is responsible for that. Blue skin like a Na'vi; Na'vi DNA here. A bit smaller, more human-like eyes than Na'vi eyes; human DNA. More muscular; human DNA. Tall; Na'vi DNA.
The reason that the Na'vi are so tall in the first place is because of the low gravity on Pandora, which does not inhibit their growth to about 6 feet like we are on Earth.