I'd love to volunteer to make and configure such hint videos.  I'd happily donate for this cause too. 

 That would be impossible if you mean a different hint for every question in the Math. For every quiz, there are at least 10 different questions; making a video for all of them will be impossible. Khan Academy has thousands of quizzes in math.

It will still be too much, even if there is one hint per quiz. Let's estimate there are 2000 quizzes (pretty sure that's an underestimate) in math. Times that by 5 and you will get 10,000 minutes of hints. For Khan Academy's small team, that is still impossible.

Also, hint videos would be super unpopular. Khan Academy's recent videos (before 2 years ago) usually don't even have 10,000 views on Youtube. Hint videos will most likely only get less than 1000 views each.

There is no point since 5-10 seconds can not teach anything. To learn a subject, you need to watch 20-30 minutes of videos. It will literally be useless.

@Dwayne, I think what they’re trying to say is that the hint videos included when you ask for help on a question are just too long to keep users engaged. A short less-than-1-minute video is easier to pay attention to and quickly understand.
Also, hint videos would essentially be clips of the videos we already have that only show what’s necessary; it wouldn’t be to hard for KA to add some iframes that start the video at a certain point.
Also, that’s the point of the hint videos; hopefully you’ll never need them, but they’re there in case you do.

StuffStash is right.