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Because Europe didn't know about America until 1492.

Or more accurately, as that wikipedia page mentions, March 1493- when Columbus got back.

Columbus discovered America in 1492.

Arguable, some think that Viking explores like Leif Erikson discovered America first.

http://en.wikipedia.org/wiki/Leif_Erikson

Yes - I didn't mean to imply Columbus was the first one, but regarding the question of why America was not on maps before, Columbus' discovery is the relevant one.

Possibly Polynesians might have landed in California earlier than the Vikings:

http://www.sfgate.com/news/article/Did-ancient-Polynesians-v...

There was a documentary about this too which raised some interesting points.

There is one more potential location they would have sailed to - Chile in South America:

http://www.mnn.com/earth-matters/wilderness-resources/storie...

And the Native Americans arrived thousands of years earlier. But it's kinda irrelevant to the question of why European map makers didn't have the Americas on their globes until post-1493.

i hope it was a joke :) Upvoted you back base on such hope :)