system
Given the following post, respond as if you're commenting on Reddit:
human
[Half-Life] Why doesn't the Combine just exterminate the human race?

In the half life series, the Combine have destroyed the vast majority of the human race. The planet has undergone total ecological destruction and humans cannot reproduce due to the Suppression Field the combine have installed. The few 'official' humans that remain live in numbered cities e.g. City 14, City 17.

The combine obviously want to discover the portal technology, but why do they maintain a civilian population? What good do ordinary humans serve to the Combine? The combine soldiers created out of humans are only used to police humans and are unlikely to be used elsewhere. Earth is it's own self contained environment in that regard.

As for the 'negotiated surrender', what is stopping them from breaking it? It's not as if Breen has any power. It's clear the combine want to kill humanity, but why not just get on with it?
gpt
They don't so much consider the remaining humans to be their enemies as they consider them to be their property. The Combine more or less think of them as livestock, so they aren't going to just throw them away.  They aren't the type to be wasteful - for example, consider how they often repurpose existing structures, or how they're slowly stealing the entire ocean.  Even the resistance cells may have been thought of more as "problem animals that need to be broken," rather than real threats.  After all, they did conquer Earth in only seven hours, and that was twenty years ago.  Plenty of time for the occupying Combine force to fall into a sense that humanity has been thoroughly "pacified."

Of course, once the Freeman returned and catalyzed the Resistance, the Combine probably reconsidered this stance.