We aren't.
EDIT:
"There is scarcely anything so exasperating to me as the idea that the natives of this country have no sense of humor and no faculty for mirth. This phase of their character is well understood by those whose fortune or misfortune it has been to live among them day in and day out at their homes. I don't believe I ever heard a real hearty laugh away from the Indians' fireside. I have often spent an entire evening in laughing with them until I could laugh no more. There are evenings when the recognized wit or story-teller of the village gives a free entertainment which keeps the rest of the community in a convulsive state until he leaves them. However, Indian humor consists as much in the gestures and inflections of the voice as in words, and is really untranslatable."
From an anthropological standpoint- two reasons.
that it is very helpful to portray a group as "other" if "they don't act like us", so saying a group is unfeeling, humorless, or conversely, overemotional and passionate can make a distinction between them and us. See the American portrayals of Japanese or Germans during WW2, or Irish during the Famine.
A lot of the early interaction with American Indians by whites was in a hostile environment, where empathy isn't really an asset. Nor is showing fear or pain. Later, not showing emotion is a common coping strategy for groups who fear reprisal for their words or actions. See diaries/memoirs of American black slaves, or 20th c POWs. (Same concept behind the myth that "black people don't feel pain like white people", which apparently is still widely believed in the medical field.)
This is a link to a really interesting pamphlet written by a Smithsonian anthropologist in the 1930s- who was frankly surprised by the depth and beauty of the Seneca Iroquois culture he documented. It's definitely not without some temporal bias (meaning academic language and standards were different in the '30s) but interesting.