See my comment above, but also, keep in mind that channeling the arcane is not the hunter's strong point, therefore would be one of the weakest attacks he (or she) has.

"Why didn't we just get him to enchant all the bullets."

For the same reason the door to Cho'Gall's house magically stops working once 10 or 25 people have gone through it. It's a game.

You also have to keep in mind it was a joke answer. Another way to say, "Who the hell knows?"

Why don't Hunters just tie explosive charges to all of their arrows?
Why don't Hunters coat all their arrows with poisons?
How do Hunters make traps that explode...with frost?
How do giant, flaming, stomping Core Hounds camouflage themselves?

Did you know that "a Mage did it" is synonymous with "suspension of disbelief"?

For my Night Elf hunter, I've always considered that he dabbled a bit in magic with most of the other Night Elves back during the pre-Wellsplosion days. He never really got that into though, magic just wasn't that interesting. He was far more interested in spending time out among the animals, tracking, hunting, learning. He knows enough about magic that he can do a few minor tricks, like enhancing his arrows with arcane energy, make them explode with fire or making a trap that will turn someone into a block of ice. After the Wellsplosion, he dabbled in the druidic arts too, since that's the way his people were going. Though this did appeal more to him than arcane magic, he ultimately didn't follow the druidic path either. He's taken little bits of those teachings to strengthen his connection with animals, to imbue arrows with deadly poisons and even to help those around him resist the negative affects of nature magic. (I've got better nature resist than a druid! Go figure.)

That's my own little story about why my hunter does the things he does, but it's obviously not official. :)