Why is it so hard to figure out who wins in the modern day wars the U.S. fights?

The U.S. is leaving Iraq. But nobody knows who won. It would be almost impossible to determine who won. We'd have to wait years and years and see how Iraq turns out. We don't know if the Iraqi troops or police will be able to provide security. The U.S. is facing the same situation in Afghanistan. We won't know who won. And even after years and years it still may not be clear. Vietnam was similar in that sense, too. WWI and WWII had definitive endings. But modern day nation building wars are so messy and they have no clear objective. Why is that? And how can you possibly fight a war with no clear objective? How can your troops leave a nation if you're not even sure what the result of the war is? Don't the troops deserve to know the results after years and years of fighting? This makes no sense.