It's only the same as a really long shutter time if the subject is stationary, and in the case of Earth-based astrophotography it never is, because atmospheric

It's only the same as a really long shutter time if the subject is stationary, and in the case of Earth-based astrophotography it never is, because atmospheric turbulence is always moving the image. If you take multiple separate shots you can combine only the good ones: https://en.wikipedia.org/wiki/Lucky_imaging https://en.wikipedia.org/wiki/Lucky_imaging