Because at a minimum, reporting it later can get the flame war stopped. Yes, it's best to stop it early, but later is better than not at all. And it alerts the mods to the degree of the problem and points out potential problem posters to them.
The mods have said many times that they'd rather have too many post reports than too few.
Fair enough.. But I never heard a mod say that - but if thats the general opinion then I dont mind posting more reports.