First, the refund they give you is preliminary.
Second, there is a common scam that debit cards are susceptible to and credit cards are not. It goes like this: I steal your debit card info. I mail you something trivial. A trinket. Cereal box stuff. I send it USPS Signature Confirm. It has "Reward Dept" listed on the outside and you sign for it.. I mean, why not? You're perplexed by the contents. Somebody spent $3 to mail THIS?
I charge your debit card. Hundreds of dollars. Not usually thousands, not big enough to make FBI radar screens.
You call Bank of America. They give you a preliminary refund. They investigate. They call me, the perpetrator, as part of the charge back process. I say "Oh, well, I have A. Brown's signature from USPS, as you can see right here."
They take the money out of your account. They call you. You are incredulous. You ask for a supervisor. You demand "So if I buy a laptop and they mail me a brick, you can't help me? Just because I signed for it?" They explain that "How would we know? You'll have to sue them to get your money back."
You see, there's a very big difference. With a credit card, the bank is on the hook, until you pay your bill. Until you pay, the fraud is perpetrated against THEM. And federal law gives you right NOT to pay a disputed charge. With a debit card, you are the victim. Hell, the bank even made 2-3% on the fraud.
Not to mention, as I replied above, I make between 2 and 3% cash back each year on my credit cards. We're not rich, but even at our spending, that's a nice chunk of money each year. Far better than the measly "points" some debit cards offer.
(Edit: And in the same vein, once you pay your CC bill, you lose most your rights and conveniences for dealing with fraud. That's why it's important when using things like BoAs ebill initiated payments to still review your statements before you pay the bill.)