?
Maven repositories are traditionally just file systems organized by convention, either local or exposed using a simple web server, so a lot of repos get this "for free" by just enabling directory listings for URLs. ProGet works a bit differently and simulates some of the traditional maven structure, so it's not necessarily trivial for us to implement this; that's not so say we can't do it, just that it wasn't deemed essential.
While I don't think that this is formally part of any maven specification, a lot of other repositories do it, so it's been on our "maybe" list for some time; we just haven't had a lot of interest in it so far, so it hasn't made it onto the roadmap.
We'll reevaluate what it would take to do it, and add it to the roadmap if we think it's something we can work in soon.