If insiders have done fuck all except blow money up billionaires asses, who can blame voters for liking outsiders?
Yes and no. Of course he has power and connections, but to his followers he is an outsider (although I don’t understand how they ignore everything that shows the opposite) and the media and many politicians treat him (rightfully) as a pariah. That gives him legitimacy in the eyes of his or undecided voters.
Now, nothing he proposes will change anything for the vast majority of his voters for the better, in fact it will make things worse. And that’s ignoring everybody else, especially migrants. But when he says “I see your problems, voters, here’s the solution: fascism”, they more readily believe him because he is perceived as an outsider, not like politicians who frankly haven’t done shit for them.