We added this feature in Drupal 7 but it has fallen into disrepair.
The main reasons for this as I see it:
1. Core developers (and contrib developers too mostly) never use it.
2. It's not up front in the installer, which it would/could be if we'd added module browser in core.
#2311691: Updater UI for installation is broken is a critical bug with the functionality.
Because we could release Drupal 8 without this feature, then add it back later, I'm opening this issue for removal. We don't need to do that now, but should do so if we get to the last few critical issues against core and that one is still not fixed.