What people don't realize is that the Cushitic language family (excluding Beja, yes they are Cushitic, but they split even earlier) is about as old as Indo-European.
So Cushitic languages can be almost as different from each other as German, Russian, Persian, and Spanish are from each other.
It is easier to learn Arabic than to learn other Cushitic languages for a Somali native speaker.
Nevertheless, the biggest Cushitic language: Oromo, is relatively close to Somali, but many of the other ones aren't.