The more you roast the beans, the less terroir you taste, so you can use cheaper and less uniform beans. In Starbucks case this has become a brand identity, so it has persisted past strictly practical concerns. Many people dislike the kind of acidity and strange flavors you can get from lighter roasted coffee, at least in the milk drinks they want to sell, and light roasts are also harder to get perfect and consistent, so that is another point against it (or the same through a different lens)
Personally I prefer a light or medium roast over an Italian or French roast, but many people disagree