Depends really. In my mum's village when she was a little girl she did. But when she came to England, even after 30 years, every now and again if she's carrying a really heavy box she'll carry it on her head. I do the same a lot. Its actually using the body's alignment to your advantage to carry things much heavier than you. Because all your big muscle groups are stacked on top of one another and the object is at the very top, so weight is more evenly distributed than carrying it in your arm, and it feels lighter. So its less of a tradtion or anything like that but more that its quite economical and practical. But yes when in I was in Kisumu, my little cousins, would carry water from the fountains alternating between their arms and their heads. And even women in the markets, sometimes have a piece of cloth made into an 'O' shape on their heads to make carrying things on their head more comfortable. This is just my experience of it anyway cant speak for everyone. But seriously, next time you have something really heavy, try carrying on your head, its like 10 x easier!