The problem is that the .pst is a single file, a rather special database file that contains the emails and their attachments, avast has to extract that infected email and attachment and put the database file back together without damaging it. That isn't simple and it may that avast aborted any change as it may corrupt the .pst file, effectively resulting in total loss.
There are many AVs that would simply delete the infected file, the .pst file with disastrous results. Under normal circumstances avast would simply be moving an infected file to the chest, now it is entirely possible that it could move the whole .pst file to the chest, which you wouldn't like. Also there is a file size limit and the chest is also limited in size so that may be why nothing was moved as the pst file is huge. Check the avast Program Settings, Chest, where you can adjust the sizes as required.
So I hope you can see as onerous as it is finding them manually is preferable to the possible loss of all your emails.
It is unfortunate that these emails weren't detected prior to your installation of avast. I'm afraid there isn't a simple solution.