Suddenly a great company of the heavenly host appeared with the angel, praising God and saying,Why did a whole company of angels show up to the shepherds? It doesn't say they sang. It says they praised God. Why did this ad hoc praise service get shown to the shepherds?
“Glory to God in the highest,
and on earth peace to men on whom his favor rests.” Luke 2:13-14
The angels are always praising God. Why did this one instance get revealed? Would the shepherds not have listened to one angel?
So many questions. I have only one answer. I think God realizes that when you are God, you do things like only God can. This is a God-sized event, His son coming to Earth. Yeah, in case you've forgotten, that was kind of a big deal.
And I also imagine that, if there are sign-up sheets in Heaven, this event needed a few of them. When one comes to realize just how big God is and how worthy He is of praise, why wouldn't you sign up to bring Him praise?
Perhaps that is the better question.