Because you're thinking of "space" as just boring orbital stuff. Astronomers think of our solar system, deep space, distant stars... a bunch of gadgets blocking the view of that is a legitimate concern.
It's not like the constellation move in unpredictable manner. The orbits are known, the movements are predictable, surely with some processing it shouldn't affect astronomy too much? If astronomers can overcome random atmospheric dispersion, surely they can elimitate starlink from post-processed images too?
