PBE is kept alive so that it can be used as a second step in the development process. There should be an internal dev server that they can use for risky changes like changing or cleaning up code, then they push things to PBE as a collective unit and for users to find the bugs while the dev team works on other areas or has meetings. It's meant to kill two birds with one stone and keep the process moving along, while also not having to hire hundreds of bug testers.
Yes, unfortunately the PBE is not a place where all bugs are fixed because there are so many of them. There's always a priority list and if the devs can't reproduce the issue within 1-2 minutes, they'll move onto another bug in the list. It's up to the users to compile the bug report properly, which is why the PBE forums have a template to use and ask the user to be as specific as possible. They'll check off bugs from the list but keep priority bugs on their mind to sink more time into when they can spare it.
TL;DR: PBE is kept alive to help RIOT save time and money, while also keeping development moving. Not all bugs can be fixed quickly and devs rely on the users to compile a proper report, which does not always happen.