Because really, "Why isn't life fair?" is not a valid question. "Life" is unique. Why are two unique items not the same? Because unique and same are antonyms.
The 7yr old part is the context for my initial answer from my world view.
No matter how hard we try, it's impossible to make life fair for our twins. We do our best to treat them fairly, but while they are the same age, they are not the same. They are not the same physical size and they have different mental strengths and weaknesses. They are not evenly matched in all/many areas. That can make things feel unfair.
On the other hand, maybe it is fair, we just do not have enough context to do a proper measurement.
