Just my thoughts... Think about it. Trump has been famous a long time. He was very popular and yet until the "tv show" The Apprentice, and then running for president, I've never heard one report on him exhibiting this "childish" name calling behavior. As long as he's been in the public eye, I would have thought some tabloid would have exposed this. And yet nothing. It's just my opinion of course, but I believe he was honing his larger than life persona on The Apprentice. Seeing what was and was not acceptable in preparation for his polarizing personality performance as a president who needed to stand out so the normies would tune in to politics. Knowing that at first they may not like him, but all the while, little by little, he could win them over with the truth.
Like him or hate him, everyone knows who he is and listens to what he says. What other former president can you say that about? I can't personally think of anyone I've ever followed so closely. Even my seriously misguided family members who are liberals, pay attention to him. I can honestly say no one in my extended family ever followed politics before Trump.