At one time, society was very stratified.
At the bottom were serfs, usually with a first name only.
Peasants were free men, not tied to land.
Next up were yeomen, once defined as a free man with an income of 40 pounds per year (from land).
Next were gentlemen, who needed to make 100 pounds per year. They were often armigerous, and this level in Scotland were considered untitled peers. They were entitled to be called "Mister."
Next, you have the peers, with addresses and styles according to rank. A prince, princess, or royal duke is a "highness," while a monarch is a "majesty," in the British system.
This system has gone by the wayside for those who are not peers. The use of "gentleman" after one's name ended around WWI, and the use of "Esquire" is in decline today, and it's use much disputed. "Mister" has come to mean any man (with Mistress and Miss for women). Peers are still styled and address in the old way, but I expect they get lots of junk-mail entitled Mr. So and So.
A president is not royal. He is basically a commoner elected to the post. The US does not have a system of titles, so, there is really no title to give him beyond "Mr. President."