I was about to say that a better reason would have been: Because it's a stupid question! Suppose the Sun got something in its eye and started blinking at one Hertz. Earth would be thrown into half-second paroxysms of daylight and darkness... No! The Sun's radius is > 2 light seconds, so we'd just see expanding light and dark bands on its face, essentially invisible at interstellar distances. A blinking extended source would need to be flat and face-on to us ...
Wrong again! Imagine a "point source" of intense bursts of neutrinos, say, which "illuminate" some large object from behind (from our viewpoint). Then the object will appear to flash pretty much all at once, regardless of its size and shape.
Only if the point source is far away from the object it's illuminating - otherwise there'd still be significant path-length differences (imagine the center of the Sun doing the blinking - the center would appear to blink at quite a different time than the limb). But then, if that's the case, the point source must be either very tightly beamed and/or be emitting ridiculous amounts of power. So I think the astronomers had it right. But maybe I'm still missing something. --Joshua Zucker