Right you are. But it need not be that way. To get a good grasp of, say, chemistry, you really do need four years of study.
But to get a good grasp on how to teach, one semester should do it. That's it. During my first year of teaching I relied on a little 20-page booklet called "Teaching Effectively". No jargon there, no fads. Just tips on how to manage student behavior, how to make each lesson interesting, and how to question effectively. It was really all I needed.
Yep, no argument there. Interestingly, many high school science teachers are teaching outside of their area of study... In my case, I have more mathematics and computer modeling (applied mathematics) background than any 4 year degree and it comes in the areas of chemical thermodynamics and reaction engineering. My point is that the certification reigns supreme and the actual knowledge of science is secondary.