Airlines don't charge for carry on luggage, but most airlines do charge for checked luggage. While I understand the desire to raise more revenue, I don't understand why an airline would want to encourage carry on luggage, in preferrence to checks luggage. Planes are built to carry checked luggage, as long as someone checks luggage you still have to keep all the fixed costs associated with having checked luggage, and carry on luggage can reduce passenger satisfaction, reduce safety and slows down departures and arrivals. Carry on luggage also increases TSA burdens and security line lengths.
Why don't airlines increasing ticket prices and then offer discounts if you check luggage rather than the full allotment of carry on luggage, instead?