There's also the issue of Anthro, "the First Boy" living only 18,000 years ago, but [writer Grant] Morrison's acknowledged that it doesn't make sense; it just had to be that way for the story.
Indeed, this is true of all fictional "cavemen"--i.e., all Early Modern Humans who lived from 35,000 to 10,000 years ago. It doesn't matter if they lived in Africa, Asia, or the Americas--which would've made them black, Asian, or Indian, respectively. Creators invariably depict them as white.
As respondent #2 tried to do, you can explain why any particular tribe of cavemen is white, even in the Americas. For instance, a lost tribe of
And this is merely a subset of a greater problem. Most humanoid aliens (Star Trek, Star Wars, the Legion of Super-Heroes) are white. If they have blue or green skin, they still have Caucasian features. Most lost civilizations (the Inhumans, Wonder Woman's Themiscyra, Aquaman's Atlantis) are white too. Even fictional creatures like fairies, trolls, and mermaids are white.
Can you imagine a race of aliens who look Chinese? Or a lost civilization of the Inuit? Or a black mermaid? You probably can now that I've mentioned them, but did you ever think of them before? Or were the fictional beings you envisioned inevitably white?
Apparently writers can't imagine that the Americas were inhabited for 10,000-plus years by dark-skinned Paleo-Indians. It doesn't fit their mental map of human history. To them, the continent was more or less empty until the Europeans arrived. Indians appeared out of thin air to oppose the colonizers the way supervillains pop up to fight superheroes.
This is an example of several things: How people unconsciously express their white privilege (white skin as the default for every unknown race). How people are in denial about the racial reality. How people view Indians as inconsequential or invisible--good only as enemies in 19th-century Westerns.
For more on Paleo-Indians, see Were Indians "Colonists" Too? and Did Indians "Colonize" America? For more on the Bruce Wayne series, see Indian in RETURN OF BRUCE WAYNE #4.
Below: A real (reconstruction of a) Paleo-Indian...
...and Anthro, the white boy who should be a Paleo-Indian.