Every culture marks the passing of a year somehow. That is not a coincidence — shared ritual does something specific to how groups hold together.
Ritual makes memory stick
Events that break the ordinary rhythm of a week are encoded more strongly than the days around them. A birthday is a reliable, annual excuse to build one of those markers on purpose.
Belonging is built from turning up
Shared attention — a group of people all directed at the same person at the same time — is one of the more reliable signals of belonging we have. It is also the thing that is hardest to fake, and easiest to feel when it is absent.