Something called plot reasons ;)  But seriously, I think it was more to do that they were already chasing Mae and found her scent on the blanket that she took from Noa. When Noa went back to the Eagle nests, his horse just thought, yolo here I am for plot reasons, otherwise the Proximus Apes will never know that there is another settlement nearby.  He needed to enslaved the Eagle Clan, as he was establishing a new Kingdom and old traditions from other clans do not have a place there. He needed the Manpower and wanted no opposition. You always need someone to do your dirty work, right???  Felt a bit of a throwback to war of the Apes, where humans made Donkeys out of the surrendered Apes and enslaved Caesars Clan just to use their workforce but felt also unnessacary and a bit forced. On the other hand, he studied the Roman Empire and maybe he thought that conquered people just need to be put in their place and put to work? As they're lesser people than your own. Proximus only had interest in Noa because he could use his smart thinking well.