Because, as Occam’s Razor Explains, He is a Liar
The NYT has compiled a list of Trump lies since the beginning of his term. Note that these are not spin, these are out and out falsehoods that are factually inaccurate.
T r u m p ’ s L i e s
Many Americans have become accustomed to President Trump’s lies. But as regular as they have become, the country should not allow itself to become numb to them. So we have catalogued nearly every outright lie he has told publicly since taking the oath of office.
Of course Trump lies about being a liar. It’s what liars do.