It is just that we are born with a sin nature because we sinned in Adam. That’s the whole concept. Adam was the perfect representative, not just for himself but for the whole human race.
Whether we understand being in Adam through federalism, realism, or different theories, nevertheless it’s because of my unity with Adam and corporate solidarity with Adam that Adam and his progeny were punished with a corrupt nature.
The corrupt nature is the punishment. What we call “original sin” is not the first sin, but the result of the first sin. That’s passed to the whole human race, and God was never more just than when He made that judgment upon the whole human race.