Why are there so many street beggars?
The unemployment numbers in South Africa are relatively high. There are also fewer opportunities for South Africans. This being the case, you will, unfortunately, find street beggars at traffic lights, (aka robots).
For many folks, this is not pleasant to observe. However, it is a fact of life in South Africa.
Many street beggars are entrepreneurial too! You will find those who offer to clean your windshield in return for a small payment. Do not accept window cleaning at an intersection. It is illegal. Simply way no thank you, and wave them away.
You may also find people who try to keep the intersection crime-free. (While rare, petty crime may occur at intersections in big cities. It is rare because locals know not to keep valuables within sight in their cars.)
Some people say that the worse thing you can do is to give money to street beggars because in doing so, you are perpetuating the situation. That may be true but we take a different approach. Instead of giving money, we give them food. If street beggars bother you, consider buying a few extra things when you get groceries. As a foreigner, you will find food to be relatively cheap in South Africa, and a few extra bananas can ease an empty stomach.

SOUTH AFRICAN STREET ARTISTS
A lot of South Africans make opportunities for themselves by creating beaded and wire arts and crafts, which they may sell on the side of the roads. Often, there are organizations that help to export their orders.
Some of these wares are impressive and are definitely worth a look. There is beautiful African wall art, jewelry, wire-beaded animals, and much more. By buying these things, you are helping to feed a family, and you are supporting an honest living.