Why does the military refer to kilometres as klicks?
It’s just a time-saving abbreviation. It also avoids the possible confusion over the various possible pronunciations of the longer word (see earler thread on that).
Same reason when I lived overseas we never said, “It’s about 50 kilometres down the road.” We shortened it to, “It’s about 50 k’s down the road.”
Convenience.
Start saying “kilometer” but cut it short abruptly (swallow it) after pronouncing “kil-” and you can see how “klick” probably came about. It’s a nice sharply defined sound that makes it easy for the listener to feel certain he heard correctly.
(Or if that’s not what you were asking, then this: Same reason you call a gymnasium a gym – it saves three syllables.)
It’s an abbreviation, easy to say, not likely to be confused with something else. K, (kay), the other main civilian abbreviation, can also be used for 1,000. The military doesn’t like using the same word for two meanings, even if they are unlikely to be mixed up.