All data online seems to indicate there is little difference between an internist and family practice. why then are there two separate diciplines?

The patients seen: Many people can understand the role of an internist (internal medicine) as a "pediatrician for adults." Just like pediatricians only see children, internists only see adults for their medical problems and have 3 years of post-med school training only with adults.Family practitioners are trained in adult medicine, pediatrics, OB, even some surgery; less training in adult med but treats whole family