Coz we have been arabized due to islam spreading into somalia and with it the arabic culture. Go to any majority muslim country anywhere in the world and you shall find those people share some cultures with arabs. Does that mean,they are not african or asian? Nope
Majority muslim countries your speaking of dont share any cultural similarities with the arabs, Indonesians, Malaysians, or even Pakistani, afghan, Iranian culture is very different to arab culture. Different clothing, food, etc.
However somalis eat what the arabs eats, dress like arabs, and other cultural similarities.