For the same reason they show a maximum of 5 FC. The pool industry refuses to recognize the connection between CYA/Stabilizer and the ability of chlorine to do it's thing sanitizing the water. Others here can give you the scientific details if you want, but lets just say CYA locks the ability of chlorine to sanitize. The more CYA you have the more chlorine you need to keep in the pool to keep algae at bay. I took over my pool with a CYA of about 250 (CYA tests above 100 are just a guess, not very specific contrary to what the pool store says). With a CYA of 250 I had to keep my chlorine level at around 20 just to keep algae away.