Because I know enough not to write prime testing code that resemble anything like that loop to have to care about reducing its search space. If you actually want to test my knowledge about prime number, you can ask and I will tell you about using some probabilistic choice instead, and that I know fast deterministic one might exist, but I am not up to date on the state of the art.

If I have to write the loop above, I am assuming it is the Fizzbuzz equivalent of your company to show that I know how to write a while loop. I am not thinking about reducing the search space because I am writing the code semi-unconscious and frankly just want to get to the next question.