Making hard cheese kosher involves full time attention from an observant Jew. And it is this observant Jew who must add the enzyme rennet to the mixture when it is at the coagulation stage. No other person is allowed to be substituted. In small operations or in times of past history, this was a purely manual operation. However, now with cheese production being highly automated, this task of the Jewish mashgiach may only require the pressing of the button on the machine. Regardless of how small a duty this is, it represents the essence of the dietary laws passed down, and must be followed for the cheese to be certified.
It might be for this constant attention and extra cost that most hard cheeses are not kosher-certified. And many of these manufacturers will run kosher productions periodically as needed.