Because that would make God a liar.
The heavens declare the glory of God, and the sky above proclaims his handiwork. (Psalm 19:1)
By studying the heavens with many different branches of science, we know that the universe had an initial beginning and was created out of nothing. Isn't that what the Bible says? The heavens also say that the universe is approximately 14.5 billion years old.
Scientists -- both believers and non-believers -- have studied the earth and have found it to be about 4.5 billion years old.
God has revealed to us his glory through the splendor of creation. The Bible says nothing about the age of the earth. But, as the Bible notes in several places, God also speaks to us through his creation. And his creation irrefutably indicates that the earth and the universe are much, much more than 6,000 years old.
The heavens declare the glory of God, and the sky above proclaims his handiwork. (Psalm 19:1)See, we're looking at it from the perspective, "why would God trick us into thinking that the Earth is really old?" But that question presumes our examination of the Earth comes from the skeptical perspective of asking of science. Think instead of the question, "What is God showing us through the structure of the Earth?"
But I've got to tell you, nothing sounds like a mind closed with a death grip when instead of stating, "That would seem to contradict this verse of scripture?" (or God forbid, phrasing it as a question like, "How do you reconcile that with this verse?"), you jump straight to saying, "because that would God a liar." In a sense, you are asserting, "You can't possibly have had a thought I didn't already have, so if you say something I wouldn't've have said, you must be perverting the truth!"