Except for when hackers stole everyone's credit card and login details yesterday.
A couple points you need to take a hard look at
1) Banking services and credit card companies themselves get hacked, providing thieves with cc numbers and account information. This is news to you? Everyone with a credit card that has done anything online with it is at risk. This is not restricted to Steam
2) I use Steam. And they don't have my credit card number at all being that I never purchased anything through Steam. But since my cc company has undoubtedly been hacked once upon a time (or twice upon a time), just like everyone else's has, I am at risk. But not because of Steam
3)
the fact that Steam accounts where hacked does not turn Steam into Malware. It makes it a vulnerable data mine just like countless other things you probably use every day without thinking twice about it