Why do people idle their engines?

It seems to be common knowledge now that car emissions hurt the environment. I’ve also heard that it has been linked to asthma and even cancer. Given this knowledge, why do people persist? Is it to avoid stressing the engine? Or laziness? Even disregarding the health factor, what is the purpose? Sometimes I go into the grocery store and come out a half hour later to find the same car sitting there wasting gas. I just don’t get it.