because your brain is the only organ that doesn't repair itself. cheap helmets can split on an impact that higher end helmets will get a decent scratch from. also, higher end helmets have better padding (back to the protection thing) and are often lighter, which means less neck strain.
I use an oneil and my oldest son wears a matching oneil. my stepson has an oneil as well, my wife has an HJC. we also have a mohave and some off-brand i picked up from tractor supply that is DOT rated as spares. my youngest son doesn't ride yet, and it looks like riding may not be his thing (he's a mommies boy, what can i say). my step daughter wears her brothers oneil when she rides. she prefers her moms hot pink HJC, but i have to make her switch when i see her riding with it on. helmets that are too big are just a couple steps shy of not wearing one at all.