Why is that? It is trivial for an overzealous prosecutor to indict someone, especially someone as hated, and for whom a suitably deranged grand jury can be assembled. I can think of at least half a dozen charges that can be thrown at him. None of them may even be true, none may be provable in a real court trial, but indictments against a private citizen are a piece of cake.
It would be the lifelong dream of a New York prosecutor to indict Trump, and let me assure you, they will try, especially if he loses the 2020 election.
They have to be alive to indict...