Mostly because of convention that the single identifier be a single string from the user. You might be able to get away with it e.g. in your home grown app but try to land that change in Chrome and you'll get pushback for not being standards compliant.

The explicit way to define a v6 URL was first defined here https://www.ietf.org/rfc/rfc2732.txt and is to use []'s around the address portion.