As others pointed out, they're soaked with grease. In fact, the problem is so bad that enough pizza boxes bundled with clean cardboard has spoiled entire ton-sized bundles of cardboard shipped to China.

Cardboard is valuable to recycle, but our inability to remove contaminated cardboard from the recycling stream has really caused major issues. Because of that, China has stopped accepting paper and cardboard products from the US for their recycling operations.