Disbelief and normalcy are a hell of a drug. Unless someone fully expects something terrible to happen and they are deliberately trying to find out when it'll happen, most people will happily ignore all of the signs until it's too late.

Just look at the lead up to the Ukraine invasion, the US intelligence services were practically screaming from the rooftops that it was happening. Russians were obviously stacking up on two borders. Meanwhile, reporters were on the street asking people how they felt about the oncoming invasion and they all said some variation of "they've been threatening that forever, it's all talk, it won't happen".