Why can't we tackle?

[Biggin1985]

I am not an NFL football player, however, I did play pee wee football, junior high, and high school. And at all levels one thing that we were taught was to not try to tackle with just our arms. If you attempt to arm tackle, most often, someone will just run right through those arms. It's clear to see so many of our players just trying to tackle using only arms.