One thing about credit cards, debit cards and checks is it allows people to keep most of their money in the bank until they actually use it. If the banks push too hard, consumers may start living like my parents did while I was a child. When the paycheck came (actual check, no direct deposit), my father would cash it and take about a third to a half as actual cash, leaving the rest in checking to pay bills. He almost never used a credit card. The fractional reserve banking system functions much more smoothly if people aren't holding onto large blocks of cash for weeks or a month at a time. The banks start being able to lend money only based on the lowest monthly balances rather than counting on people drawing down their accounts a little at a time. Consider that there is only about $800 billion of US currency in circulation (http://www.newyorkfed.org/aboutthefed/fedpoint/fed01.html), with the majority of that held outside the US. If every worker in the country started sitting on $1000 cash outside the banks, that would consume a large amount of the currency in circulation. It might even wipe out currency available in the US.
Oh, woe is you when you try to cancel them. Cutting them up doesn't do the trick. Calling and canceling doesn't either. I've had a card company rep argue with me over the phone that I could not cancel, even though there was zero balance. I did eventually get that one canceled but what a hassle.
Sit down, right a letter to each one, provide your account number and your signature, send it via U.S. mail, "return receipt requested", ask for a letter from them via U.S. Mail, verifying that they have indeed canceled it. By law they have to provide that upon request.
If they tell you over the phone that they have canceled it, chances are great that they are lying to you and you will learn of it the next time they send you a solicitation to buy "credit card protection" for your account, mentioning the last 4 digits of the account you thought you had canceled.