A lot of people have already mentioned this, but the short answer is that you ultimately need a civil time that approximates local solar time. For example, you

A lot of people have already mentioned this, but the short answer is that you ultimately need a civil time that approximates local solar time. For example, you want to make sure that day changes happen at a time when there's little activity. Civil time doesn't need to be exact to solar time, and there's room for a fair amount of divergence, but when the divergence gets too great, people will spontaneously create their own civil time instead of using the official civil time (see, e.g., western China). It's better to have a minimal set of official civil times that are good enough for local purposes than have a large set of unofficial and semiofficial civil times.