That's incorrect.
Rennet is
complex of enzymes produced in the stomach lining of mammals.
There are many methods of producing these enzymes, which (btw) constitute less than 0.1% of the final cheese.
One of these methods, likely the one your research lead you to, is taking the freeze-dried stomachs of calves and chemically and mechanically extracting the enzymes. While perhaps not palatable to some vegetarians, it's certainly not cut and dried (haha).
MOST cheese today, especially domestic cheese (and all kosher cheese), uses rennet from non-animal sources, mainly produced by specific molds or fungi.
According to the government, about 60% of domestic hard cheese was made with genetically engineered chymosin in 1999. The number today is undoubtedly MUCH higher. This specific source of this is typically a genetically engineered strain of the common fungus Aspergillus niger.
There are many other sources of non-animal rennet (or rennet substitutes), so the amount of hard cheese produced domestically from animal-rennet in 1999 was SIGNIFICANTLY below 40%.
Of course, in an irony of ironies, the "genetic engineering / GM foods are bad" lobby consists, in my own personal non-scientific experience, mostly of vegans and vegetarians. So, no using genetic engineering to get that fungus to produce chymosin, because, well, that's just WRONG.
I stand corrected! I knew it was the enzymes that is used but I thought it was found in the animal's stomach lining therefore the lining itself was used. Either way, I thought the percentage of rennet use in cheese was higher because other methods proved to be much more expensive.