Disregarding the possibility of life elsewhere that couldn't possible survive on Earth discounts most of the planets we know about.
So if survival of those life-forms depends on gravity, pressure or chemistry then we definitely should not rule out places where those are different than on earth, in fact that is to be expected. But temperature is a very important factor and the make-up of the star itself is also very important.
So it makes sense to check the likely places first before spending time and effort on much more unlikely places.
Some of these organisms also have novel ways of acquiring energy, so it seems like research like this is probably our best near bet for understanding how life can thrive in extreme conditions.