We have other ways of solving conflicts.
First of all I want to say that I'm really quite excited at this opportunity to contribute to your understanding. Alternatives to violence is a topic that's very dear to me, and I understand that you want to know why we have chosen not to create a species more effective at warfare, despite having the theoretical ability to do so.
Second, before diving into the answer, I want to admit I'm afraid you might not like this answer. Because in order to understand it you will have to let go of one fundamental assumption that is currently taken as a fact by virtually everyone in your society (based on a cursory glance on your most popular entertainment, newspapers, advertising, religious texts, etc). I therefore ask you to suspend any judgements until you have finished reading. If after this you find yourself triggered, that's ok! Perhaps it will be easier to read at a later time, maybe in another hundred years or so.
Now, the assumption I'm referring to is the idea that violence is an effective means for creating lasting solutions to conflicts. Our assumption is quite the contrary that violence in any form, although sometimes necessary, always leads to more conflict. How did we get there?
Just like yours, our history is long and full of stories of wars and victors. Long, because it takes time to get to the point where one could create a species for a specific purpose without it going completely sideways. Full of wars, because all civilisations naturally goes through a phase where the ability and technology to control and kill develops faster than the ability the communicate and empathise with each other. So for some time violence might be unavoidable, and from that perspective undefeatable armies must seem like a valid strategy. At some point, however, the question arises if there might not be an alternative to an endless cycle of wars.
So we took a long hard look at our history and asked: what was all this violence good for? Not even the greatest victories attributed to the longest lasting peaces (what's left of your roman empire today?) have lasted in the long run. It seems quite pointless and hopeless, but, being giraffes, we know that behind all actions there's a life-serving need. What needs might we be trying to meet through war, bigger armies, and more deadly weapons? Is it security? Stability? A need for food and water? Obviously, to some extent, peace and harmony? And haven't many conflicts started because of a need for understanding and mutual trust? How many civil wars haven't really been about respect, identity, and belonging?
Surely, in an age where we have the technology and resources to design custom species at will, we can find alternatives to solving conflicts that are more effective (and more fun!) at meeting these needs than deadlier armies. Exactly how that is done is more than I have the energy to go into today. Perhaps it can be left as an exercise for the reader to come up with? As a starter, just consider what would be possible if you instead of 100k elite soldiers had at your disposal 100k elite mediators, empaths, communication experts, counsellors, negotiators etc.
Hopes this provided the understanding you were looking for!
A baby giraffe