They were not lying. Their perception of reality was skewed. The eyes of man take in the world around him, and only then is he to put upon it his own personal stamp. The eye is meant to precede the Peh. But some people reverse the order, seeing life as they perceive it to be.
Modern man assumes as a matter of course that the world follows a natural order of cause and effect.
From every corner of life he gathers evidence that worldly efforts are guarantor of true success. He speaks too soon. Having opted for a life where man controls his own affairs, everything he sees supports his preconceived notions. Were he to adopt a more spiritual perspective, he would see the Hand of G-d behind every occurrence. As the Meraglim, he can no longer see G-d's promised land.