Because, in the words of his ex Chief of Staff, he's scared s-less.
"“He’s scared s—less,” Kelly told The Washington Post. “This is the way he compensates for that. He gives people the appearance he doesn’t care by doing this.” ...
... “For the first time in his life, it looks like he’s being held accountable,” Kelly told the Post."
John Kelly: Trump is ‘scared s—less’ | The Hill