Omni 1

I find it real interesting that Mosiah, -

"...warned of the Lord that he should flee out of the land of Nephi, and as many as would hearken unto the voice of the Lord should also depart out of the land with him, into the wilderness—
13 And it came to pass that he did according as the Lord had commanded him. And they departed out of the land into the wilderness, as many as would hearken unto the voice of the Lord; and they were led by many preachings and prophesyings. And they were admonished continually by the word of God; and they were led by the power of his arm, through the wilderness until they came down into the land which is called the land of Zarahemla.
14 And they discovered a people, who were called the people of Zarahemla. Now, there was great rejoicing among the people of Zarahemla; and also Zarahemla did rejoice exceedingly, because the Lord had sent the people of Mosiah with the plates of brass which contained the record of the Jews.
15 Behold, it came to pass that Mosiah discovered that the people of Zarahemla came out from Jerusalem at the time that Zedekiah, king of Judah, was carried away captive into Babylon."  (Book of Mormon | Omni 1:12 - 15)

Sometime after Lehi died...I would guess around 580 B.C., Nephi and his followers move to what they called the land of Nephi.  Some 450 years later, Mosiah leaves the land of Nephi and came to the land of Zarahemla.  The land of Zarahemla and the land of Nephi must not be far from each other because we real later that there is much travel between the two.

But for some 450 years, the Nephites knew nothing of the people of Zarahemla.  Knowing this, why could there not be many other people throughout the western hemisphere that lived here and never met.