Because we allow you to use the credit card without using the chip. You buy from a compromised merchant, or a merchant that isn't properly hashing it's credit cards? bam, your card is stolen. You put your card in a compromised card reader? your card is stolen. With a chip, (a smartcard credit card, like you use in Europe) this is way harder.
Yes, yes, in America, our credit cards now have chips, too... but they also have the old magstripe, and everyone buys things from the internet anyhow, so we're giving that number out to everyone.
The other side of this is that the laws in the US are heavily in the customer's favor. It's a pain in the ass, you have to complain about the charges, but it's very rare that we are actually responsible for any of the charges. (The merchant who accepted the stolen credit card is left holding that particular bag.)
Because of this? Personally, I don't worry too much about it. I figure any one of my cards might be out of commission at any point of time, and periodically I'll just call my credit card company and tell them to give me a new credit card number, just so I have a fresh one that isn't floating around all the darknet sites. I figure a credit card being compromised every five years or so is 'normal'