There is actually a popular book that answers this kind of question: “The Dictator’s Handbook: Why Bad Behavior is Almost Always Good Politics”

I haven’t read it, but from summaries, my understanding is that the answer to your question is that every high ranking office depends on maintaining support from other people with power. Even if we disregard voters, Trump (and other presidents) need to keep generals, oligarchs, and senators happy, or they can remove him from power. Naked disrespect for the rabble displeases these people, because they’re still sensitive to assassinations, market disruptions, challenges from other power centers like state governments, etc.