UA bumped me over to AA this afternoon when it became clear UA couldn't get me from DCA to MEM tonight and staring at my AA BP for the last few hours I've wondered about this.
It was "quaint" the first time I flew AA in my adult life (around 2003 maybe) and now it seems positively a historical artifact. They're using the same IER/Sato/etc SITA-compliant thermal printers UA (and just about every other airline in the developed world) is using so it's not a "capability" issue. They've vlearly tweaked the template to e.g. add TSA PRECHK, seating group numbers, the cabin name, etc. So it's not something so baked that it can't be changed, and as SITA-compliant it should be relatively easy to change the PECTAB... So there must be a non-technical reason for continuing to use the ATB2 stock two decades after the introduction of eTicketing and over 10 years after IATAs "simplifying the business" inititaive phased out paper tickets for all but extremely rare cases.
Don't get me wrong, I love the look of ATB2 stock -- it "feels" so much more official than the UA/WN stock, bit it also feels a bit like an antique.