Why are South African mountains short and flat?
I suspect you are referring to the mesas and buttes dotted around the Karoo - the large semi-arid plateau that makes up much of South Africa's interior. These are a product of the glacial and geological history of the region. The flat hills are capped by hard, resistant dolerite. This is solidified lava that was forced between the horizontal strata of the sedimentary rocks (which make up most of the Karoos geology) through high pressure. The dolerite erodes much slower than the surrounding rock and forms a flat cap above the underlying rock.

Most South African mountains are actually not like this - Google "Drakensberg" and you will see that the country's most prominent and highest mountain range is far from flat and short!