Why is delta 0.5 at the money?

Could you please explain why delta is 0.5 at the money for an option contract? I understand that delta represents the change in the option's price relative to the change in the underlying asset's price, but I'm not clear on why this value specifically equals 0.5 when the option is at the money. Is there a specific mathematical or financial rationale behind this phenomenon? It would be great if you could elaborate on the concept and provide any relevant examples to help me better understand.