Donald Trump’s so-called “war with the media” is a good example of what philosophers mean by an over-determined effect: multiple causes, any one of which would be enough to support it.
Which is to say this “war” (a clumsy and lazy term) will almost certainly continue, despite the periodic discovery by journalists that Trump loves to banter with reporters, and that he lives and dies by the very media coverage that he poisonously calls fake.
The campaign to discredit mainstream journalism is thus a permanent feature of Trump’s political style. Why?
1. Because it’s a base-only presidency with a niche, not a broadcasting strategy.
"Americans are used to candidates who, when they win the nomination, try to bring the party together by embracing those who supported the losers. They are used to nominees who, when they win the White House, try to bring the country together by speaking to voters who did not support them in November. This is normal behavior. This is what they expect from presidents of both parties. Trump rejects all that. His idea is to deepen the attachment between himself and his core supporters so that nothing can disturb that bond."
Attacking the national press corps makes good sense, what with the Republican base in a permanent state of rage at “cognitive elites.” It makes even better sense for a President with a base-only strategy, a decision reflected in the polling data. About a third of the country is with him:

2. Because this is what they have; they don’t have much else. The Trump presidency is a shambolic mess. As Charlie Warzel and Adrian Carrasquillo of Buzzfeed observed, hating on the media is its most consistent theme.
"Trump has been clear on one issue: the untrustworthy “fake news” purveyors of the media. As he’s struggled to even put into motion the kind of sweeping legislation he promised on the campaign trail, Trump’s relentless focus on the media has been the only constant amid the disorganization. Nine months in, it seems clear that Trump’s only real ideology — and the only true tenet of Trumpism — is to destroy what he believes is a deceitful mainstream media."
3. Because Trump is a creature of media— and its creation. Josh Marshall recently pointed this out: “For all the purported hatred of ‘the media’, the main Trumpers are almost all fundamentally media creatures. They think in media terms. They are media creations.” He’s right about that.
"Trump himself is a self-creation of the 80s and 90s New York City tabloid culture. His comeback in the early part of this century was driven more than most people understand by the success of The Apprentice. Why else do you think people in the Philippines or Kazakhstan paid millions to license Trump’s name? It was the brand driver of the licensing empire which allowed Trump to become the 45th President."
Steve Bannon was a publisher. Before that he was a movie producer. Jared Kushner bought a newspaper and used it to fight his battles in the press. On down the list they are all media people. They don’t hate the media. Indeed, they can only understand most battles in media terms. It’s all they know. They aren’t prepared for anything else.
4. Because people in the White House think “media” warring is governing.
When CNN fired three journalists and retracted a story about the Russia connection, White House staffers were said to be “elated.” By something CNN did. This is weird.
"As he escalates his attacks on the “failing media,” Trump and his allies are increasingly convinced that recent evidence, including the retracted CNN piece on an aspect of the Russia investigations, will prove to skeptical voters that the mainstream media has a vendetta against the administration."
It’s one thing to strut about calling the news media the opposition party. That’s good theatre. It’s another to think you’ve banked a win when the press corrects itself.
5. Because turning reporters into ritualized hate objects is easy to do, supporters love it, and it meets Trump’s need for public displays of dominance.
The press is not popular; we know that. Attacking journalists is a freebie: no price is paid. These things were true long before Trump announced. What he adds is a specific-to-Trump neuroses: his need to dominate others, which was visible in his only full-on press conference as President. Josh Marshall again:
"We’ve collectively been living in Donald Trump’s house now for more than two years. We know him really well. We know that he sees everything through a prism of the dominating and the dominated. It’s a zero-sum economy of power and humiliation. For those in his orbit he demands and gets a slavish adoration. Even those who take on his yoke of indignity are fed a steady diet of mid-grade humiliations to drive home their status and satisfy Trump’s need not only for dominance but unending public displays of dominance. He is a dark, damaged person."