I think a few years ago I could have written a comment like this, but now I've been a teacher for a couple of years now.
It is hard to teach something. Being able to do it is isn't anywhere near the level of expertise needed to:
* Do it up on a board at a moderately quick but consistent cadence, with engagement and narrating the whole part through.
* Not be taken off track by inevitable distractions.
* Understand the stack of necessary knowledge in the ways needed to figure out what a student is missing.
* Completely reinvent your lesson plan when you find the students unexpectedly lost, providing whatever the missing bit of knowledge is to get your class back on track.
* Handle unexpected diversions and requests for deeper knowledge on the fly.