If you see a marker... 50% chance it's dead, 25% chance it's barely alive, 15% chance it's legible from the back row, 10% chance it's fresh and usable the way you describe it. It's impossible to tell before you try.
After you're done with it, you need to remember to close the cap, or find a trash can to chuck it into (and make a call if the marker kind-of-still-works-but-not-really). Seems like a small thing, but adds mental overhead. And it's a big enough overhead that a lot of people don't do it. Otherwise, how would we end up with the distribution above in the first place?
Now teaching/explaining is a performance art. Doing the "wait, let me try another marker, is it better now?" dance ruins the show.
That's why mathematicians prefer chalk. The subject matter is already difficult enough, there's not enough bandwidth for all of that nonsense. Have chalk, will write. End of story.