The word "have" is used in two different ways. One way is to own or hold something, so if I'm holding a pencil, I have it. But another way is as a way so signal different tenses (as in grammatical tense) so you can say "I shouldn't have done it" or "they have tried it before." The contraction "'ve" is only used for tense, but not to own something. So, the phrase "they've it" is grammatically incorrect.