Because the syntax looks somewhat like the function notation in math, and a variation of it is also present in many programming languages, often functional, but also Java since version 8. It's quite widespread and not very bizarre / specific to JS.
You'd have mentioned tagged templates [1], I'd have to bow.
[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
It counts, because it makes it familiar to many people.
Had JS introduced this syntax without it being anywhere else I would be completely with you: it would just be weird and unnecessarily complicated.
I find this syntax very clear too (though I still prefer function () { ... } for most functions). It has the merit to make some code using small functions, often somewhat functional, lighter to read. But this is a matter of taste, of course.