Harvested Sorrow
I think it's due to the most popular religions ('the big three') being based around worship. People assume that all other religions are like theirs and based on worship of a god of some sort.
That's sensible, and because of the prevalence of Chinese culture in contemporary Western civilization, most people's exposure to images of the Buddha are of the Bodhisattva Hotei who, while he wasn't Śakyamuni, he was still very popular.
