Not all companies that sell products in California include the Proposition 65 warning labels on their products. For example, small businesses with 9 or fewer employees may be exempt from Prop 65 requirements.
Alternatively, a company may not know that they are required to add the warning to their products and are at risk of being sued. Products that do not display the label may still contain toxins on the Proposition 65 list. You can find updated lists here.