Because he has no respect for the tradition or the prestige of the office, and never has.
He ran for President as a lark in the past, made a semi-serious effort of doing so in 2015 and discovered that tapping the inner hatred and fear of change among voters (combined with a terrible D candidate) and voila! The office of President fell into his lap. He never has been a serious student of politics and the art of compromise; he became President and was handed all the trappings of the job but really only wanted the power, not to be Presidential. He as an adult had no one to answer to; no shareholders, no family or others so the only way he can be President is to do just like everything else in his life he has ever done. He has never had to fear loss or the word no, for there was always another sucker to fleece, another bank to swindle, another scam to work. He is in his own mind a worldly man, but he has no background in anything but "deal-making". His moral convictions are based on sand, his word is not worth the toilet paper he wipes his ass with, and by most counts, he is a failure in everything he has ever done, and a failure as a person.
But none of that matters, because he found out that he could con 30% or so of the population + a political party that had the gravitas and the means necessary to promote and protect he and his family.