In an interview in 2017, when asked why he had not filled many State Department positions, Trump infamously stated, "My opinion is the only one that counts." He still believes that and that belief will be his downfall. Trump is alone because his world view isolates him and his worldview values only him. His narcissism does not allow for other viewpoints. People who express contrary views are shown the door, either quickly or in a short while.
Trump looks for views that reinforce his own opinions. Sadly, those views come mostly from right-wing conspiracy theorists. He re-tweets them, he copies them verbatim in his own tweets. He even hires Giuliani who himself based his Biden ideas on a conspiracy theory book written without access to critical facts.
Such an approach to the world both discourages others from engaging him in dialogue and accounts for his reality-free daily recitation of alleged facts. If he were not president, no one would give credence to anything he says.