You must understand that not everyone has or can get credit cards.
Consider that those who are in the the lowest 20-30% of income tend to have fewer credit cards (or none), and lower credit debt, although some have quite high credit card debt relative to their income. So you really aren't comparing the same demographics (the population of all income earners, used to calculate average income, and the population of all credit card debt holders, are not the same groups of people). Once you remove those folks from consideration, then credit card usage may still average higher, but accept that it is unusual for people making less than $20K-30K/year to have much credit card debt.
You must understand that wealth and income are two very different (although related) concepts.
One must note that there are millions of people in the U.S. who have wealth; they have net assets of over $1M (excluding their homes). Many of those folks have assets greatly exceeding $1M. And although it might seem foolish to carry a large balance on their credit cards, they may have quite low interest rates, and simply find it simpler and more convenient to use credit cards in lieu of personal loans. Suppose you have $2M in net assets, and want to buy a classic car or a diamond necklace. Charging $30K and carrying the balance until a dividend check arrives may make sense.
Understand also that not everyone makes the same choices, or good choices.
Carrying a credit card balance may appear like a poor choice, especially when you are not wealthy, or have lower income. But suppose you have a high credit limit across several cards, and you need to handle a short-term financial challenge (car repair, layoff, medical bills, etc). You might use the credit card to pay for that purchase, essentially financing an extraordinary event over a longer period of time. And although having a balance of more than 5-10% of your monthly income may seem foolish to some, it may make sense to others.
And some people choose to carry balances of 50% to 100% of their credit limit. Others realize that keeping their credit utilization below 30%, 20%, or 10% of the credit limit is a better plan (both interest rate and risk wise).