This is a byproduct of the market DESIGN in aerospace. By having a duopoly rather than either a monopoly or wider competition, you're forcing the margins down to near zero. This means that there is always a CLEAR replacement option for every product made by Boeing or Airbus, and the customers (the airlines) know it. If you had more players (embraer, gulfstream, bombardier, china?) you could have better competition, but there are few large body jet providers in the market. Granted this problem stems from the airplane companies own strategy of trying to get airlines to go single model (all 737s for example) to save on repair/maintenance.

Economic theory wise its a fascinating market issue because you have clear view of the buyers and the incentives in the marketplace (also both big players get huge govt subsidies).