It is almost impossible to lose as an incumbent President.
The only reason it happened in 92 was Ross Perot.
And that guy still hasn’t apologized for what he did.
While Democratic donors spread their money between 23 people right now. Trump gets most/all Republican donors $.
By the time the Democrats get down to one nominee, Trump will have a much bigger pool of money to play with.
That is why it is so much easier for an incumbent.