Because this actually works in the current version of Arc. Maybe not quite as concise as the proposed syntax, but probably as close as you can get without adding syntax (at least that I can think of).
So you need to weigh the cost of adding more syntax to the language for the savings of "$1" vs. "(_ 1)".
Note that the latter is more than twice the size of the former, and that you'd also need to put the arguments to [] into a list prior to passing them into the fn. I'd say $1 is a pretty big win in terms of conciseness.
