Why is the national anthem played at sporting events?

Perpetual instilling of nationalism in the subjects of the Vaterland.
Just like having the picture of past Führers in the classroom, pressuring/forcing young impressionable minds to parrot the “pledge of obedience allegiance” in school, and glorifying the Wehrmacht military, going so far as to allow recruitment on school grounds.

Here ya go.

National pride

But why at sporting events and not at movies, plays or concerts?