Every good teacher you had in your life knew much more than the level at which they taught you. You can't teach effectively if you don't master flawlessly the subject you teach. You achieve that level by going way beyond what students learn.
At the basic education levels, this is easy to achieve. At the University level, you can only guarantee this elastic push by heading into research.
By this logic, algebraists and combinatorialists also can't teach calculus, given that they probably only got as far in calculus (namely 1'st year of grad school) as all the non-tenured, non-research adjuncts.
Yep, they can probably not teach you much about any non-trivial problem in calculus, nor would probably engage you much in current advances being made in field. They would make fantastic teachers for teaching Algebra-I and II.
Some of my worst courses have been Field theorist teaching electromagnetism.
At the basic education levels, this is easy to achieve. At the University level, you can only guarantee this elastic push by heading into research.