Why as a jungler you should never help any laner to win a lane

Because we all know how it goes.

No matter how much you help anyone if they are a worse player than their opponent they will still lose.

And if they are better than their opponent then they will win even if you don't help them and even if the enemy's jungler help them.

Don't try to explain it to them because they won't understand. Be the change you want to happen on the Rift. raptors -> Krugs -> Red rinse and repeat.