Because (so say the flat earthers), the sun is only 32,000 miles away and so behaves like a spot light :-D.
Which of course doesn't take into account the fact that you would see the sun behaving in very strange ways, and you'd have to create an entirely new branch of physics to make it work, something they've all failed to do.
If you really believe on this, I recommend you to visit a good Psychiatrist!