3 things make walking less dangerous than cycling.
1) Speed. i don't know about you, but i cycle faster than I walk. therefore any accident is likely to happen faster, and likely to hurt more.
2) we share the roads with cars. who travel very fast. if cars were driving along pavements regularly, perhaps peds would wear protection.
3) contact points with the ground: the amount of surface area of tyre in contact with the road is many times lower than the area in contact when walking. even the sole of just one shoe is many many times larger.
But anyway, you seem to be missing the point again.
I dont think any real cyclist expects their helmet to prevent a MASSIVE head injury.
It'll just help a bit if you topple off your bike and knock your head a bit. not that we're expecting to fall off, or that we somehow posess lower bike handling skills. helmet clad cyclists are just as optimistic and skilled as non helmet clad ones. Helmets are just a precaution.
Let me ask you a simple question.
If you had to headbutt a wall, Would you rather do it with or without a helmet?