It's not just not planting deep enough. It's also planting with the root pointing down. If you germinate in a paper towel, you should put the seed in the soil with the root pointing slightly up or to the side. I've seen a time lapse somewhere showing what happens when the root starts out pointing upward. As the root starts out upward then bends down, it gets leverage that sort of rotates the seed up. That twisting through the soil helps get the seed shell off. If the root starts out pointing down, it just pushes the seed straight up, making it more likely for the shell to stay on. Of course you also need to have enough soil for the seed to push through, so planting too shallow is also a problem.
A picture is worth a 1000 words: