Do I really need to replace my helmet every 2-3 years if it hasn't been in a crash? OK, I know from the top answer here: When or how often should I replace my bike helmet?, that I should buy a new helmet every 2-3 years, even if it hasn't been in a crash and shows no signs of damage.

But can someone explain to me why this is so? I'm not trying to be a cheapskate, but just want to know why the helmet would degrade just with regular wearing? Do motorcyclists replace their helmets every 2-3 years too?

And don't tell me that this is what the helmet manufacturers recommend. Of course they will say it should be replaced even if there is no visible damage. Reminds me of the adage: "Never ask a barber if you need a haircut".

Appreciate any input/insights you can give.
The Bicycle Safety Institute disagrees with the 2-3 year rule. They have a good page on replacing helmets.

I usually replace one when the foam rubber gets old and crumbly, when the adjustments quit working, or when I damage it. UV can deteriorate the plastic if you leave it outside a lot. And, if you ever "use" a helmet, i.e., crash and save your skull with it, you should replace it because the styrofoam and outside plastic can weaken.