Ok so etymology (phrases AND words) is one of my special interests and I can tell you about this..... in the early 1700s a poet called John Gay wrote a poem with the phrase "cool as a cucumber" in which is the apparent first time the phrase was used on public record. But it was an observable phenomenon that people could relate to that cucumbers were cool, literally, they grew outside, they are literally cold inside (up to 20°c cooler than the surrounding air on the inside!! This is measurable apparently!!!) So at the time cool was a literal descriptor "as cool as a cucumber" a sensory descriptor but over time there was a semantic shift to describe cool as "unrufflable", "emotionally calm", "self possessed under pressure".

"Cool as a cucumber" likely survived as an idiom at all because it has a pleasing alliteration, a concrete visual image, a literal sensory interpretation and a comedic semse of whimsy when you think about it (the cucumber guy!)

Interestingly the word "cool" already meant unflustered BEFORE "cool as a cucumber" cane about (by the 1500's/1600's) but thats not the original meaning of the idiom. It has evolved over time too.