There was an unwritten rule in nature: if you wanted ownership of a piece of territory, you had to hold it against the attacks of other races. Only then would other races recognize your claim to that land. Otherwise, they could attack you again at any time. In other words, if the ogres wanted other races to recognize their ownership of this new territory, they had to repel the humans head-on and secure their rule over this land with blood and death. Otherwise, the struggle for this territory would continue for a very long time.
There were many unclaimed lands in the wilderness.
No one really cared if a small group wandered about, but once a large population migrated, it would disrupt the existing balance.
Humans would not allow a large Ogre Tribe near the roads unless they had been beaten into fear and chose to avoid this territory, clearing out monsters elsewhere and opening another route. Most roads between cities had no clustered populations because the large groups that once occupied the areas around them had all been cleared out by generations of humans. Trade routes had been carved through the wilderness in just such a bloody fashion. Wild creatures remembered this lesson; even when migrating, they avoided key roads. The ogres occupying this place was a challenge.
They wanted to declare their ownership of this land to the other races, and nothing demonstrated their strength better than defeating humans.
After all, this was wilderness, not cultivated farmland. If humans failed once, they generally would not keep pouring resources into driving them out, because there was no profit worth fighting for. At most, they would establish a watchtower line along the border. But if it were land with fertile fields cultivated for generations, the conflict would become a fight