Because you say so or because you have proof?
There are many reasons, some of which I already referenced, but the #1 reason why it's the closest Aikido has come to being an effective self-defense system, is because it's the only Aikido style which successfully, and gracefully, integrated drills for dealing with retractable punches with follow-ups, into its training method, all the while remaining distinctly Aikido. It's not Aikido plus boxing, it's Aikido philosophy and design, successfully extended into a closer, faster ma-ai.
Having hard randori with random street attacks doesn't hurt, either. Well - it does literally, but you know what I mean