I don't use credit cards for the credit; in fact mine are completely paid for every statement. They are used for the customer protections and other provided "free" benefits. If some scummy or outright scam-y thing is charged to my Amex, I know I will have Amex on my side. If my card is stolen, Amex will refund any fraudulent charges and overnight me a new card; I probably won't get my debit card overnighted, though they will probably refund the fraud. The other thing is credit card points, which are essentially a benefit paid for by credit card processing fees charged to businesses. Many cards also offer access to "private" airport lounges. And other benefits I'm forgetting off the top of my head.

Additionally, having high credit limits, low usage, and older accounts improves credit scores for loans/etc.

No interest is charged if there is no balance carried statement-to-statement, so why bother with silly debit pins and such.

That's how it becomes the default way of payment; it's not really "credit".