To Coffee Roasters. Why does Starbucks over roast their coffee beans?

Actually, your last sentence answers the question.

A light roast allows the subtle flavors of the bean to show through. As beans from the same place will taste slightly different from year to year (yet alone all the different places they get them from), blending and roasting dark helps get rid of those subtle flavors for a controllable "uniform" flavor. The idea is that the coffee in Chicago will taste exactly like that in New York.

I'd rather go to a small coffee shop that has a unique taste than go to a coffee equivalent of McDonald's.

I've been trying to answer that question for nearly 10 years. I stopped drinking their coffee and roasted and grinding my own at home. And, people still buy that crap like it doesn't taste like metallic ash, just because it has the starbucks label on it. People are sheep.

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Because it is easier to burn the beans and have exactly the same charred flavor than to bother with different tastes.

Because roasting them correctly requires skill and dedication along with proper employee training and that is too expensive....