Originally Posted by
fumei
Chaldea (under Romani's temporary leadership) only acted because it was confirmed that the world would end with the incineration plot, and Servants were summoned for this purpose. Once that task was taken care of (including the aftershocks that were EoR) the world was saved from immediate destruction and there was no way anyone could've known another future-dooming event on the same scale was going to unfold in just a few months. Of course they're not just going to have hundreds of Servants just sitting around "just in case" for the hell of it, when they're already under heavy investigation by both the UN and the Mage's Association for both using pretty much prohibited magic (at least without actual permission) but also for suspiciously being the only ones not affected by a year-long time gap. It's not exactly free to maintain all those Servants either. Had they known part 2 was gonna happen in-story your argument would make sense, but to the current Chaldea's POV, their task of protecting humanity absolutely had been fulfilled.