It's an easy answer made complicated by bacteriology in general
and dairy science and contamination specifically. To put it
simply, the combination of raw material, handling, bacterial
ecosystem, and affinage make it difficult for botulism spores to
gain a foothold and to grow. Botulism requires a nutrient-rich
medium, one in which it can move, and one that has good
conditions. Those good conditions are a low salt content, a high
pH, and proper temp. Cheese matches few of these, and moreover,
cheese bacteria tend to outcompete botulism. For example, during
the initial stage, when milk is fresh, those lactic bacteria
multiply so quickly that botulism never gets a chance. And if it
did get a chance, the acidity becomes low rather quickly. And
then after the acidity, there's the salt. Botulism is inhibited
at 4-6%, and I'm talking about pretty drastic inhibition. Then
after that you're stuck with a pretty solid block of protein and
fat. If botulism is inside, it has little food. It's not like
soup or processed c