People who are good at stuff are not always good at teaching that stuff to others. Often, in fact, as someone becomes an expert at something, they have trouble viewing the first steps from the metaphorical seventh floor landing.
Try reaching a four-year-old to read “cat” or what 2+2 is and you’ll realize that things that you “just do” are actually a series of difficult steps for a beginner. In much the same way, experts “just do” play an instrument, play a sport, solve an equation, or create art.
Some of the worst teachers I ever had were geniuses, but they had trouble getting into the minds of their students, much like the xkcd comic (“silicate chemistry is second nature to us…”). Teaching is a skill; in fact, it’s a dozen skills stacked on top of each other. Thank your local educator today: the current administration continues to underfund education and the current society underappreciates teaching as a skill (and I haven’t even touched on the classroom management, lesson planning, and social skills required by most educators).