That is exactly what I entered and it was declared wrong. The "correct" terminology was: Why do "we've" to fly to China. Really? The use of "we've" is actually painful to a native English speaker. Cringeworthy, really!
Yes, it's horrible. But it's not our fault. :) Duolingo's system creates contracted versions of phrases automatically, so we don't have enter both "does not" and "doesn't", for instance. Sometimes, when it screws up, it gets really noticable.
Okay, why isn't "Why must we fly to China" accepted as it means the same thing.