That's not quite true, many helmets have only DOT designations, and were not submitted to SNELL for testing. Snell has much more stringent testing, and personally, I wouldn't think of buying a DOT only certified helmet.
How many riders replace their helmets after 3 or 4 years? That is also a safety issue. Most helmet manufacturers will (for around $100), test a helmet, replace all the liners and fabric, and even give it a new certification. In the past, while still racing formula cars, I would send my custom painted Simpson Super Sharks in every 2 years.
How many riders abuse their helmets by leaving them on the mirror or hanging fromt he bars while parked? That also causes a deformation in the lining, possibly rendering the helmet less effective?
Most everything on the body can heal, be repaired, or replaced. The brain can't. I can walk with a limp, I can't function without a brain.