Because he lies all the time. I think the average for 2018 with something like a dozen Liza day. Some of these are relatively trivial, but some of them are very important. Either way, it demonstrates that he is a liar who feels no need to talk truthfully to the American people. That's just one example, Trump falsely claimed that he did not know anything about payments two women who had affairs with him. Now he admits that that was a lie. As another example, Trump promised in campaign rallies that the Republican Congress would be giving people a 10% tax cut before the election, even though Congress was not going to reconvene until after the election. Trump is also frequently prone to exaggeration is that in someone else, you did not always stretch the truth, might be charitably written off as mere hyperbole. For example, he recently told troops that he was visiting that he had passed the largest pay raise for service members ever. That's false.