To try to answer the question, "Why . . ?" I would say that if you are buying cheese in a sealed wrapper, it would have started off sterile, unless there was lack of care when it was packed. The mould spores that started off the mould must have come from somewhere outside that wrapper, that is from fingers, from the knife that cut slices from it, from the container in which it is stored, from the air surrounding it when it is out on the table or worktop, or from the inside of the fridge.
Th solution is to keep it in the wrapper as much as you can, unless you are cutting off enough for immediate use, or to use within the next few days, and to avoid even brushing a bare hand against it. Always use a clean knife and don't leave it out in the open any longer than necessary, return it to its closed container or the rezippable wrapper in which it was bought.
If it was bought from the counter, and cut from a large piece, perhaps someone's fingers had touched it? I would only buy small pieces from the counter, which would be eaten very soon, before any mould had a chance to spread.
Incidentally, cheese can be frozen if well wrapped. Hard cheese may crumble when defrosted, but that doesn't matter if you are going to cook it or grate it. Soft cheese is usually OK.