> (including reading from leapsecond tzdb data)

That's part of it: If I were writing a standalone program that could extract info from tzdb or whatever, I'd happily jump through those hoops, and not bother anyone else's libraries. I don't really care about the ergonomics. But for JS scripts in particular, there is no information available that is not provided by either the browser APIs or someone's server. And such servers are not in great supply.