Let's talk business.
Though it appeared that humans were entirely powerless against the Calamities, the truth was much more complicated. Many brilliant minds gathered after the invasion and managed to create weapons that had the potential of pushing against the mysterious enemies, and by studying their remains, those weapons were polished even further.
There was a problem, however. The mental and physical load of operating such weapons far exceeded what humans were capable of handling. Attempts to substitute the operators with artificial intelligence always ended in failure. In the end, the only ones who could operate those weapons safely were the Valkyries, which was yet another blow to those who opposed placing the fate of humanity in their hands.