That's because the culture that existed was erased by the colonizers. The fact you consider America and its culture is only 250 years old is a great example of that.
Native american culture goes back thousands of years, and it would be commendable to develop an interest in it as a lot of people do with the Japanese.
“Native American culture” only goes back thousands of years if you blend all of them together. At that point there isn’t much of a meaningful difference including the culture that came from the early Spanish colonies.
It’s not clear to me one tribe that wiped out a bunch of others to become powerful enough to be dominant for a hundred years is any more deserving of interest over Spain. From a rarity perspective sure, but not because they have any more right to the title of “American culture”.
Most Americans are not related to native americans - why should they consider their culture as their own? It is absolutely fascinating, but a few generations back - say - Irish family would be hard-pressed to feel any belonging to it - the same way any conquerer would feel about the conquered.
Native american culture goes back thousands of years, and it would be commendable to develop an interest in it as a lot of people do with the Japanese.