You don't see warnings on every item because nobody has bothered to do the stupid tests yet.
Anything will "cause cancer" if your idea of causality is an observation of cancer in a small-N, high dose uncontrolled study in inbred rodents. Prop 65 doesn't bother to incorporate basic toxicology - dose and exposure need not be considered.
If you drink coffee, you should know that has acrylamide in it. If you regularly consume 100,000x the dose from a cup of coffee, then you'll be at risk of developing cancer. But, your coffee shop needs to display a Prop 65 warning because CA's legislative warriors encouraging the toxicological equivalent of denying evolution; denying that the dose makes the poison.