Not all Somali men have weak jawlines and these problems are actually common in all ethnicities. As @Halimo Supremist pointed out eating soft food can cause a weaker jawline and to strengthen the jaw, you have to exercise by chewing gum regularly.
I've been doing the exercises for awhile myself and seen a massive improvement. It works and can make a difference to everybodys facial asthetics. A common misconception is thats its genetic. Its definitely not.
Look at pics of somali men from back in the day. They all have prominent jawlines because they chewed harder foods.