Keep in mind that (particularly with the fat "M" South African plugs) the plug might actually be safe at 30A or 40A rather than 16A, but 16A is the highest useful rating (because of wiring or other limitations such as the contact pressure in the female socket), so that's all they bother to test for.
(The M plugs are sort of like a round-pin version of the huge UK mains plug and are enormous in comparison with most other mains plugs used worldwide).
As @Neil_UK says, the limitations are a product of a committee that comes up with requirements for safety including temperature rise, fire retardant characteristics, and shock safety and markings. The requirements are typically codified into a general law requiring conformance (National Regulator for Compulsory Specifications Act) and the actual technical testing requirements are determined by specified groups (and revised from time to time, without there necessarily being changes to the law). The actual testing can be done by whichever laboratories are accredited, for example, a UK laboratory could be used, and they will test to the required standard laid out by the South African authorities.
In this case, it appears that the relevant standard only applies up to 16A, so there is no point in testing for more.
The requirements may seem a bit arbitrary, but in virtually every case there will be sound reasons why every one of the requirements are in place, and they will be agreed to by industry and government representatives with the aim of assuring safety yet not making the product too difficult or expensive to produce (and sometimes as a non-tariff barrier to protect local companies, but that's another story).