Because the light we use to measure the distant galaxy's intensity is light that left the galaxy long ago. The cosmological redshift of this light tells us the epoch when the light was emitted, because of the wavelength shift that occurs due to the stretching of space during the journey, but the intensity of the light tells us nothing by itself. At a given wavelength, the brightness of the distant object depends on the brightness of the light in the galaxy's rest frame that is red shifted into our detection band pass. If we know the kind of object it is and its spectrum, we can approximately determine its redshift from its apparent brightness in two band passes. This is what astronomers are doing for very distant galaxies. But it is only an approximate method. Apparent brightness, of its self, does not tell us where a galaxy is NOW, only how far its light has journeyed.