Otherwise you couldn't drop the braces like you currently can when passing hashes to functions.

    x, y = 1, 2

    puts x: x, y: y
    # ==> {:x=>1, :y=>2}

    puts x:, y:
    # ==> {:x=>1, :y=>2}

    puts x, y
    # ==> 1, 2

Which would be inconsistent and annoying.

(and knowing Ruby, I imagine there might be some syntactical ambiguities as well)

{ x }, depending on where you put it, could be a block that returns the value of x.

  5.times { x }

This could either call the given block 5 times, or pass a hash of :x => x as the first argument.

I guess because keyword arguments couldn't use the same shorthand if they did.

Because unlike js, ruby chose to use {x, y} as the set literal.

I definitely like the clear visible syntax