I think that's a leftover from typewriters, the pairing of () with numbers existed there, and [] wasn't available. See [0] for an example. [0] https://en.wikip

I think that's a leftover from typewriters, the pairing of () with numbers existed there, and [] wasn't available. See [0] for an example. [0] https://en.wikipedia.org/wiki/IBM_Selectric https://en.wikipedia.org/wiki/IBM_Selectric