Both The World and The Fool ‘don’t have a care in the world’ because they are spiritual. The World is spiritually mature and The Fool is innocent of the physical world because he is just starting his physical experience at zero.
The World Card. A spiritually mature human repels trouble like a street-wise person repels muggers. ‘Been there, done that, know how to deal with it.’ So The World can be oblivious to problems and no harm will come. With a strong immune system, somehow you get no symptoms.
The Fool Card is a congenital optimist. Facts do not get in his way. He is free without being free of anything specific.
The Fool is when you don’t let reality get in the way, when you just let it fly, are ‘up’ and optimistic … and oblivious, and free-spirited. The Fool depicts that floating-on-air feeling that messes with your mind when you are in love. The Fool (to whom no harm comes) ‘lets it flow’ as he enjoys an absence of all problems. I think he is whistling.