We use a third party library to linkify URLs, and it doesn't handle URLs with embedded parentheses. Handling parens is actually rather tricky because you have to consider parens that aren't part of a URL (like (www.google.com)). We'd also need to integrate the change into Markdown and the editor highlighting so that there's not a disconnect between what you see and what you get (these are wildly different designs, by the way, which complicates the changes). Finally, Dani also doesn't like me fiddling with those libraries because then we'd be locked into a specific version.
That's why it's the way it is presently. ;)
You can fix it by using the Markdown link syntax directly rather than depending on the linkify feature: