Learn Dutch: Waarom draag je niet dezelfde sokken? - Why are you not wearing the same socks?

Because sokken (socks) is plural, and all plural nouns in Dutch are de-words. We use dezelfde for all de-words and plural nouns, and reserve hetzelfde exclusively for singular het-words.

Since sokken doesn't have an article here, why do we negate with niet instead of geen?

We use geen for indefinite nouns (like Ik draag geen sokken for "I am wearing no socks"). However, the word dezelfde acts like a definite article, making the noun phrase specific. Because we are talking about specific socks, we must use niet.

Why is niet placed immediately before dezelfde sokken?

By placing niet directly in front of dezelfde sokken, it specifically negates that part of the sentence. You are wearing socks, but not the same ones. In Dutch, niet usually goes right before the specific object phrase it contradicts.

Could I use the verb aanhebben instead of dragen in this sentence?

Yes, absolutely! For wearing clothes, Dutch speakers often use the separable verb aanhebben. You could perfectly well say Waarom heb je niet dezelfde sokken aan?