Because then the "education majors" would actually need to know something about the subjects they teach. Most math education, physics education, and/or chemistry education majors DO NOT know sufficient about their topic areas to actually teach it effectively.
I agree, my daughter is going back to college to be a teacher and I was so surprised at the amount of her subjects that were going to be education, child psychology, child development, etc. Not a log of education there as in subject matter, I mean.