We aren’t the only ones to do that. The Germans do as well, for example: guten abend and gute nacht. It’s just a convention. Besides, one definition of evening is “the period from sunset to bedtime”. So if people are up and socializing (and saying “good evening”), it still technically fits since it’s before their bedtime. I suppose there’s a differentiation just so there’s more clarity in a conversation. Of course, we could all start a movement and try changing the meaning of “good night”. :o)