Why are the sites in North and South America with claims for great age all on the East side?

Taking Meadowcroft, Topper, Pedra Furada, the Clovis sites in the
Chesapeake area as being evidence, why are there no comparable sites
on the West Coast and the Great Plains? The first three claim age
beyond that claimed for the paleo-Indian migration, the Clovis sites
are on the "wrong" side of the continent, engendering theories of
cross-Atlantic migration.

Is there any logical explanation for this or is it just chance that
the migrants traveled until they ran out of room?
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