Why do so many people write macademia?

Because the ear finds a word it already knows. Said at speed, mak-uh-DAY-mee-uh sits very close to academia, and once you have heard academia inside it the e is impossible to unhear. The two words have nothing to do with each other. Academia goes back to the grove outside Athens where Plato taught, through Greek Akademeia. Macadamia is the surname Macadam, Gaelic for son of Adam, and it has an a in that syllable because John Macadam did. There is a way to catch this without knowing any of the history. Botanists build a genus out of a surname by adding -ia to it, which is where Banksia comes from, and Magnolia, and Fuchsia. When a plant name has that shape, the part in front of the ending is somebody's name and is spelled the way that person spelled it. Split macadamia after the second m and the rule does the rest.