How is america the land of opportunity? even though not everyone can get jobs or come here from other countries and work.

America was the land of opportunity, because, since most people were immigrants, everyone started in relatively similar material/social status. United States territory was not fully explored so there were riches to be found.
In "the old world" there was established social hierarchy. Your destiny was determined by your birth to a great extend.
America can be percieved nowdays as a Land of opportunity, because it's relatively easy to start a new venture, find capital to expand your enterprise. What counts is your idea, not your background.