Why is angle of attack information not displayed in the cockpit?
Many aircraft do include them, because it's very useful.

For example a modern 737:



A Garmin G1000 (one of the more common General Aviation glass cockpits):

 

Specifically the part that looks a bit like this



And a more traditional analog/mechanical gauge:



The problem is that usually when someone stalls, it's because they've made a mistake - not because they can't see the AoA. Either they're disorientated, or they've made a mistake with airspeed or an obstacle and are exceeding the aircraft's performance envelope.

Typically, the lack of an AoA dial isn't the reason you stall.