It's not just the cost of the two guys up front - it is all the support functions as well. The admin staff who makes sure all the paperwork is in order, the chap who does the check in the sim, the sim, the person who books the stopover hotel, the stopover hotel, the chap who runs the airline's anonymous alcohol abuse hotline, the chap who assesses the complaints lodged by FOs about their captains' poor CRM standards, etc., etc., etc. It all adds up.
Think of the weight penalty of carrying all that equipment up in the front of the aircraft. Think of the additional seats you could stuff in the cabin. Think of the premium you could charge for the view form the first row.
The capabilities of human operators will also be a limiting factor for safety and system capacity. Building systems 'keep the pilot in the loop' limits what functions can be implemented.