I don't think they were less advanced at all. North American Indians covers a very diverse group of many different tribes. Yes, central plains tribes were nomadic in the sense that they followed herds of buffalo but you have tribes in the Mississippi valley that are known as Mound societies and were referred to by the colonists as civilized tribes. Cahokia was home to 40,000 people and was definitely considered a cultural center. You also have the Pueblo communities in the southwest that created large structures and had trade routes.

Cortés conquered the Aztecs in 1521 and the Plymouth colony was founded in 1620. That's about 100 years in which American tribes were being decimated by diseases brought by the Europeans, so a lot of the North American tribes, by the time Europeans encountered them, had already lost a significant amount of their people.

Text books in the United States like to down play the cultural and technological advancements of North American tribes versus Central and South American tribes.