Did you know that in the South American continent, there are thirty four language families and over a dozen isolated stocks with about one thousand individual languages? Wowza. It looks like the Amazon can brag about biological and linguistic diversity. This represents the highest level of diversity as compared with any other continent. All of Africa, Asia, and Europe combined have twenty one language families. Africa, for example, the single family Niger-Congo has 1436 languages while Bantu has over 1000.
So how did this happen? Native Americans migrated from Asia in three separate waves over 10,000 years. It is important to have a good sense of time, and with that, a good sense about how long it take a language to evolve to the point that it would not be recognizable by ancestors going back five, six, or ten generations. It is amazing when you think about it.
With each large migration of Indians, spanning that huge chunk of time, they all would have been foreign to each other. Imagine the second or third wave arriving, and seeing Indians who arrived three thousand years prior. They will not have a common language. Their cultures may be very different from each other.