Well for one, Robotic warfare was something that was used in my universe, but that all scaled up out of control waaaay to quickly. before anyone realized, the disconnect had set in and autonomous robots were massacring entire human populaces, because why bother winning hearts and minds when you can just replace them. it took the equivalent of a total holocaust across twenty five planets for everyone to come to their senses when the news reported human casualties topping out above two hundred billion.

given humanity's mission and their history in-universe, there's no way they'd tolerate such unnecessary casualties, after all, their mission across untold time is to ensure human survival at all costs.

as a result androids and robots and other autonomous non-human war devices were scrapped and replaced by human infantry. as a result war casualties are high, but not astronomical.

besides any of that, the robots cost a shitload of money and resources to build, and human soldiers propagate their own existence naturally and as a simple side effect of existing.less metal and plastics are used to make one soldier and keep him up than to make one combat effective killbot.