Life in most extreme conditions

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If life can exist in the most extreme conditions on earth, why not on other planets? As this article says:

After 3 billion years of evolution, life has flowed into every last nook and cranny, from the bottom of the sea to the upper edge of the stratosphere. From blazing heat and freezing cold to pure acidity and atomic bomb-caliber radiation, there’s seemingly no stress so great that some bug can’t handle it.

It certainly can, and I wonder why some scientists reject the possibility outright.