THere's a big difference between holding a referendum on an important, but temporary issue, such as the war; and on changing the very structure of government.
We didn't hold a referendum on the war here, either, but we do when we pass amendments to the Constitution. The issue here is whether France will be absorbed into a super-state. Such matters require the expressed consent of the governed, and not merely a calculation by the ruling class that they will not be punished too severely in the next elections.