Yeah - between you and me - the real reason is like the castles, fortresses, monasteries and cathedrals of the past a stadium is built as a statement building. Uniquely identifiable as the cultural home of that tribe. I don’t think there’s ever been a trainee architect who wants to design parking garages when they grow up. Some stadiums are a triumph of form over function. Also the large empty surrounding flat areas enforce the impact of the architectural qualities by removing any outside context or reference… but you didn’t hear any of that from me.