An incumbent President has only been denied his party's nomination five times, and only once did the incumbent initially become President via election. Why are Presidential primary elections usually landslides for the incumbent if they are running?
The only way someone can answer that this is incorrect would be to argue that the proper adverb is "always" rather than "usually".
The thing about a sitting president is that in general, they've already shown they can do the job, and can get elected in a general election to do that. None of their primary challengers can say that.
In addition, a President is the uncontested leader of the party, which has very real consequences, such as appointing the leadership of the national committee (DNC or RNC) that runs the entire primary process.
Nobody is going to challenge that, unless it is pretty much certain the sitting president will lose the next general election. This last happened in 1968 when Vietnam was deeply unpopular in the Democratic party, and LBJ had approval numbers in the mid 30's.
Ford is another instructive exception. He had in fact never been elected (due to the unique situation of Watergate), and thus got a serious primary challenge from Reagan, which was not decided until the convention.
Why would they not be? In almost any election anywhere in the world currently doing the job is a massive advantage assuming that whatever you are supposed to be in charge of is not in the process of visibly self destructing.
You can claim credit for anything that's going right, blame anything that's going wrong on your opponents blocking your policy, or the previous incumbent leaving you a huge mess to clean up and travel anywhere in the country drumming up support, funds and doing rallies while passing the cost onto general taxation if you do some of the day job stuff while you are there.
Primaries are essentially elections to find out who will be in the election. It makes sense that this Incumbent Advantage is magnified, as people are voting on whether they want to make use of the Incumbent Advantage.
The general political wisdom on matters like this is: "Don't bet against a winning horse." An incumbent has already won the election once, has a proven track record and national recognition, and has clout within his party that can sway people to support him. Worse, if a party wanted to abandon an incumbent and pick another candidate, they would need to provide an exceptionally good reason; people expect an incumbent to run again, and violating that expectation could shake faith in the party as a whole and cost the election in and of itself.
Normally this is a given, and the incumbent's party holds normal primaries and allows the incumbent to sweep the field naturally. The GOP has been restricting or cancelling primaries for the 2020 election because Trump is tremendously unpopular outside of his base, and while Trump would probably still win the primaries and be selected as candidate — that's the power of incumbency — the party is worried about infighting and giving the appearance of internal divisions. They do not want to give a voice to Trump opponents within the party.