Why light rays from a distant point source , considered parallel ?
Not all lines are considered parallel, just the close ones. It is because the angle between them is very small. If you wandered why a large star looks like a point to you the answer is similar.
Maybe, another reason could be that, when we consider parallel rays after reflection or refraction, we say they meet at infinity, or at a very far point. So, similarly, the converse would be, light rays from a distant point would be parallel.