Why are native American languages similar to semitic languages?

They are not. There is no evidence whatsoever to support that idea.

It is a longstanding European and American myth popular in the 19th century. It earliest occurance was in 1650 by Thomas Thorowgood. He had never been to North America and had no evidence. It was first refuted in 1652. Later, with the discovery of large and sophisticated ruins of the mound-builders, Mississippian culture and in the Southwest, racism prevented white Americans from believing they could have been made by Native Americans. The LDS church picked up this idea and promoted it.

There are 29 large language families in North America. They are as different from each other as Indo-European, Turkic, Sino-Tibetan, and Semitic language families are from each other. There is no relation to Semtic languages.