Question

Why do rays from object placed at infinity are always parallel?

Answer 1

In reality, light rays are never perfectly parallel because the distance to their source point is always a finite distance.

However, if that distance is vast ( across the Universe, or just across a Galaxy ) then the distance traveled is vastly greater than the distance of separation of the two rays, so the rays are very nearly parallel.

So, in theory, if the distance traveled is approximately infinite, and the separation of the rays is close to zero, then we can call them parallel.