Because there is no recorded history on the origins of the many tribes of Native Americans that inhabit North and South America. Any theory like boats or walking across the Bering Strait thus can only be guesses.
another answer
Scientists are not sure how the first people came to America because the path was not preserved. For a long time the earliest discovery of human habitation was in New Mexico. People got there some way. They did not parachute from an airplane. No path of artifacts led to the habitation point in Clovis, New Mexico. I can see some prehistoric Harry Potter in Siberia saying, "Great big bird, fly me and my people to Clovis, New Mexico." No it did not happen that way! People were somewhere in North America before they came to Clovis. The question is where?
We know fishing boats existed in 50,000 b.c. which took people over at least 50 miles of water to Australia. Those fishing boats could have sailed along the shore of the Pacific Ocean establishing small fishing villages as they went. From about 30,000 to 12,000 b.c. the ocean was up to 100 meters (110 yards) lower than today. Those villages would now be covered with water and not visible. Islands 20 miles off the California coast are populated by creatures whose ancestors crossed to them over dry land. Archaeologists can not find indications of villages 100 meters under water if they existed.
There are indications that a camp in Virginia existed 23,000 years ago. It could have provided a source or people. The people there and who built the mounds in southern Illinois could have come from Europe. In 6,000 b.c. the entire population of North America north of Florid, east of the Rockies, and south of the Great Lakes was wiped out. Whatever happened destroyed traces of earlier settlements.
It would have been possible for people to walk from Siberia to Alaska over land.
The groups would have consisted of those who could fit on a small fishing vessel or those who could live by killing one seal at a time. An upper limit existed on each village.
Probably people came by several routes.
They are the English.
people crossed a land bridge
Scientists believe that the earliest humans lived in East Africa, particularly in the region known as the Great Rift Valley. Fossil evidence and archaeological discoveries have supported this theory, suggesting that early humans evolved and spread out from this area.
The belief that the earliest Americans migrated from Asia is based primarily on DNA research. DNA evidence proves people from the Bering Strait down to Tierra del Fuego are descended from native Siberians.
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nomads
The earliest inhabitants of north America are believed to have been Caucasians. These were made up of a number of indigenous people who settled in north America.
Most scientists believe that the first people to reach America crossed the Bering Land Bridge, a strip of land that connected Asia to North America during the last Ice Age. This migration is thought to have occurred around 15,000-20,000 years ago.
There is some dispute as to where the earliest people in The Americas came from. Many historians believe they traveled across a then frozen Bering Strait all the way from Siberia and then traveled down through what is now Canada and America.
scientists know that the people fannd out the north and south into mexico and central america
Not all Scientists are unbelievers. But some people can not take it on faith alone that there is a God.
I believe the Aztecs were some of the earliest people to value chocolate.