Board Thread:Questions and Answers/@comment-7657843-20161026140751/@comment-27929601-20161027223303

Meaning that only one stone resided on earth, althought it is very unlikely that another one is here. It would be too coincidental that two of the most powerful objects in the universe are on the same planet.

If the Eye of Agamotto is an infinity stone (I do believe it, but please, no spoilers) is kind of forgivable since Agamotto is a powerful pure magic being he could have obtained it, because of it being a powerful "magical" object.