It was not so much a rejection but a point he was making. He was not saying that why is invalid. rather, that without a certain context why is not meaningfully answerable for any thing that is outside your domain of regular experience (remember alien example). He is trying to show how to think and consider, that there is an ever escalating level of depth that must be considered that we take for granted and that learning how to ask the right question and introspect is key. Asking the right question is harder than getting the right answer. For a similar example, it is like asking how to use an array for something when what you really want is a hashtable or why this linear approximation is not working for the non linear data. You cannot answer the person without first explaining why what they are asking is not meaningful and cannot be answered until they first understand concept X.

The answer he gave was beyond amazing, there is an immense amount of knowledge there and I learned more from it than just physics. Just like that he gave a whirlwind tour that covered hundreds of years of physics, covering concepts from gravitation and Newton's Law to quantum field theory and does drop hints as to why magnets repel in multiple ways (electromagnetic forces everywhere - normal force and why you don't fall through floor are related - but effect stronger in magnetized iron since electrons aligned to spin in same direction).

And that is what separates someone like Feynman from most scientist. He is aware. He doesn't just regurgitate some encyclopaedic definition that is true but ultimately unenlightening, nor does he give a broken analogy when a good one cannot be given. Instead he leaves you with just the right questions in mind. He weaves an interesting story with a background and motivation and ties things up neatly in a way that you end up learning much more than what you asked for and leave with a deeper insight about everything in general and wanting more. Beyond physics even. I wish I could have met him but I was born too late. Around someone like him being ignorant would be something to bask in and enjoy.

If you read his Lectures in Physics you can enjoy at length that same first-principles, hands-on, everything-is-open-for-explanation method. It's not good as a first physics text, but really fun as a second one.