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.