Because it's true!
The decision to break up is a subtle balance of forces between the desire to stay and the desire to leave.
Missing you means that a part of him still feels what was there.
He feels a connection but this connection is not strong enough to stop him from moving forward.
The decision to break up is rarely totally black or white.
Most men will have at least some mixed feelings about this.
It doesn't stop them from eventually still taking the decision though... And going on with their lives.