That's the reasoning behind Neutron Bombs !
No more living things. Everything else intact .
Scary idea if you think about it.
Because civilians can't, won't or don't always get out of the way of the fighting in time.
Especially in conflicts like the one happening now in Gaza, where Hamas terrorists are deliberately using civilians as human shields:
Link -
Because the weapons of today are not precise enough. Although improvements are being made. Compared to WWII we see much less collateral damage today, as air strikes and whatnot became more precise. Maybe with the development of new weapons (such an EMP blasts) we could see a decrease in collateral damage but I doubt it can ever be completely eliminated where war is concerned.
Because its collaterals are awesome.