Because after moving very slowly and steadily for just about forever, the other galaxies will suddenly make a jump of like ten thousandth of a degree.
I don’t know how many light years the nearest edge of this galaxy is, but I’d wager it would take some time for the jump to be apparent to us.
Various events around the universe occur on human timescales. If time stopped for use, we would effectively skip ahead on the view of them.
I actually think we could reliably catch 1 second time stops. Scientists monitor various pulsars. They spin multiple times a second, throwing off radio wave pulses. If all of them suddenly went out of sync with our clocks, it would definitely be noticed. It might take several, however, to prove it wasn’t a weird hardware glitch.
