Because sex sells their product. They use sex because they think it is effective for them and to a large extent it is true, however it works with only low-risk products. Sex is an effective way to attract attention. People are hard-wired to notice sexually relevant information so ads with sexual content get noticed.
The brand impressions are shaped by images in advertising too. But the brands are perceived as sexy and some customers want that. Using sex to sell everything from alcohol to banking services has increased over the years as well.
The increase in visual sexual imagery over the three decades is attributable to products already featuring sexual content in ads, not necessarily widespread adoption by other product categories. Specifically, alcohol, entertainment and beauty ads are responsible for much of the increase.
But it is important to note that sex is primarily used to sell low-risk products purchased on impulse. Sex is not as effective when selling high-risk, informational products such as banking services, appliances and utility trucks.
Much of the growth has been seen in alcohol, entertainment and beauty advertising through selling sex in their advertising. Sexual content gives a purchase advantage. Products not using sex in their ads are mostly charitable organizations and computer companies. Women are used to sell products most often when pitching sex.
With the exception of entertainment advertising, females overwhelmingly occupy the pages of sex-selling advertisements. Using sex to sell products such as alcohol and tobacco is a moral issue. The upward trend in erotic ads is a reflection of society too. It takes more explicitness to grab our attention and arouse us than before. We can see during our lifetimes the changes in sexually explicit content on television, movies, books and other forms of media beyond just advertising.