Ok guys, can we leave OP's mom out of this for even one thread? This is serious.

It maintains more momentum.

Throw a ping pong ball and a golf ball, see which goes farther.

Lead being a soft material is a part of it too.

A lead bullet deforms on impact, getting wider, which causes more damage as it passes through flesh.

Kinetic energy = (mass * velocity²)/2. It's not as important as velocity, but yeah it's important. The real key is that it is soft. Meaning it will deform more readily when it impacts a target. When it deforms it spreads out, creates a larger wound channel, and more effectively dumps its energy into the object it hit target than just cleanly passing through.

That said, these days there is a big push to move to all copper bullets for hunting. They're more expensive, they don't deform quite as well, but they do consistently pedal now, they don't shed mass, and they're basically inert to our bodies. I practice with cheaper lead core bullets, but I have the more expensive copper stuff to hunt with.