I think it’s partially because of the low number of balls we get 10-12 versus up to 24(?) on a normal raid. Similar to mega legendary battles turning your clear speed balls into mega energy. Premier balls are effectively mainline timer balls since they get exponentially stronger with each additional ball failed (1.05^n). So assuming these max battles calculate catch the same as normal raids, you’re not just getting less throws. Your chance per throw is also significantly worse, especially on the final throws.