Chris, this is really easy. Your logic is that you can use your elbow to pry his head away. It's the same logic you see in action movies, were you see the hero elbow the villain in the head when he's got his back. The only problem with reality is that when you try to elbow a guy that has your back, you won't connect with your elbow 99% of the time, you'll connect with your triceps. Same with the arm triangle. He's putting pressure on your shoulder. His head is nowhere near your elbow. Leverage wise, you've already lost the battle. All you have is your triceps, with less leverage than he's got, so you're stuck. Your elbow can't reach his face because he's in too tight.
The reason the arm triangle doesn't work sometimes is because the angle has to be right, the grip, and all the mechanics. I suck at it, half the time I'm just cranking the guys neck. The other half it's tight, and they tap. But as far as escaping it, people rarely escape it when it's on.
The arm triangle is oooold..!

Before the Marquise of Queensbery in the 1800's, wrestling was still allowed in boxing, and even the boxers utilized it. (Standing arm triangles)
My best advice is to test it for yourself. have a good bjj practitioner slap it on, and try to escape! You'll see why it's hard. (Unless you have retard strength) I've found that if I can move my body from side to side, and use all my strength early on, ( I'm pretty strong corewise), I might escape it. But that's not my elbow, that's my whole body trying to get to it's feet. And even then, half the time it just speeds up the process, and I tap even earlier.