What’s up with X and Z? Why are they so seldom used? Why do they represent the foreign or “the other” to the point of being heavily used in cliche alien names?
They represent sounds that aren’t used all that often, but are still part of the language. The “z” sound actually can be represented with an “s” (e.g., amused, used, blues) and with “x” (e.g., xylophone), so it’s more commonly used than the letter.
“X” is more common than most people think, mostly because, while there are very few words that begin with “X”, there are more that have an “x” in them (e.g., example, extinct, sex).
They are sometimes used for alien names because they are rarely used as an initial letter. Most SF writers these days avoid that particular crutch, though.