man you have a twisted sense of history.
very few greek or roman wars ended when there was no one left to fight. its an extremely dumb way of doing things. it drags the fighting out for longer, meaning you take more losses. it destroys more of the land and assets, diminishing the value of what you are gaining. and without a large local population and some sort of administrative structure in place the land you took is not going to produce until you can import a population, which just causes further issues. it also tends to make the next enemy fight harder because they know you are just going to kill everyone.
even when their own rebelled it usually lead to the ring leaders being killed and then decimation (1/10) of the soldiers being killed.
Carthage is probably the worst case of destruction, but that was only after 3 wars, and an invasion of Italy, and even still they took prisoners/slaves. destruction was pretty never the first option.