Because every straightforward way of running an external command on Unix involves fork(). So someone wrote that API not thinking much of it. Then shock horror

Because every straightforward way of running an external command on Unix involves fork(). So someone wrote that API not thinking much of it. Then shock horror they realize running a throwaway command is fork()ing the main process. But now everyone is too angsty to change it because someone out there might rely on the environment copy functionality, even when they shouldn't.