Probably to keep things balanced, that you don't just plop them willy-nilly and expect your mountains of core wood to help you out without issue. Also rewards clever base design where their sightlines only contain enemies and not vikings.

If you want help in a fight and not to get attacked, try taming Lox or two-star Wolves. Of course... now that you've killed Yagluth, those pets are more an expendable distraction and less of a legitimate help.