Because you'd need code on the OS side (or at least ATM), and big corporations are more interested in respecting the user's privacy more than protecting our work.
hhp
Why foundries are not protect their fonts by some kind of buildin "serial numbers" or marks? i understand that kan't stop piracy, but kan show the source were it was stolen.. i'm askin that becouse i've meet big packs of fonts from all major foundries at peer2peer networks..