Because he's doing a fantastic job and the alternatives would range from dreadful to downright scary.
^^^ This ... just this.
Trump's no saint but he's taking his job as President seriously and he's actually doing his level best to deliver on his manifesto. He's also improving the economy and apparently improving the lives of African-Americans.
The Dems, in contrast to Trump, are showing themselves to be a deranged bunch of chancers and extremists who shouldn't be allowed anywhere near the levers of power.