I think back in the day when people still fought with swords and shields, they weren't quite so up-to-speed with human anatomy.
You'd be surprised, the egyptians knew a lot of anatomy due to their funeral rituals, the romans knew very well where liver and kidneys are, and what kind of death it will cause to stab in them. And even dark-age medieval europe had very creative techniques for torture and execution. The heart, being the best protected organ second only to the brain, isn't a primary target anyway. And almost any deep cut lethal or not will take you out of the fight in real life.
Another reason for forcing left hand shields were formations. In a formation your shield protects not only your own left side, but also the right side of the man to your left.