A piece I wrote on codesharing a while back explains this phenomenon:
The airline alliances (an extension of informal codesharing) were formed in the 1990s in an attempt to facilitate seamless travel between a potentially vast number of worldwide points by integrating the networks and schedules of 20-30 member airlines. Typically each alliance will have at least one major player in the USA, in Europe, in Latin America and in Asia plus a dozen or more regional carriers across all continents.
Often carriers "codeshare" meaning that on the ticket of a passenger originating in Des Moines, Iowa and flying with American to Chicago and then BA to London and onto Edinburgh, the whole trip will appear as "AA" on the ticket, even though most of the trip is operated by BA. Codesharing gives passengers peace of mind that psychologically they are travelling with their local carrier (and therefore the safety of their local carrier) regardless of where they are going in the world. It also gives the alliance clout in the reservation system as the routings with several codes will appear several times versus routings with a single carrier would only appear once. For example BMI's London-Edinburgh route carries codes of about 8-9 other carriers so will appear on 8-9 lines of the CRS even thoiugh it is a single flight!
The alliance model does have its flaws though. It has been built around the concept of a number of large hub airports (e.g. Dallas, Chicago, Frankfurt, Amsterdam) and is very much focussed on facilitating connecting traffic. Quite often an alliance will try and "poach" passengers from the backyard of another alliance whilst charging it's own originating passengers a fortune.
For an illustration of what I mean go onto ba.com and look at the price for a Club World ticket from London to Dallas. Now on the same website enquire about the same flight but this time start your journey in Amsterdam. Note the huge price differential! It is often up to 50% cheaper to start your journey in Amsterdam than it is in London.
Meantime KLM is offering similarly good fares to UK passengers transiting through Amsterdam whilst charging a lot more to the Dutch boarding in Amsterdam!
All that happens in the end is that everybody poaches everybody elses passengers!
Worse still the AMS-LGW-DFW passenger costs the airline a lot more to carry than the LGW-DFW passenger as there is both the additional AMS-LGW leg as well as the cost of the transit (moving bags etc).
You can now begin to see why so many alliance carriers are in such a financial mess!
Throw in Low Cost carriers into the short-haul market and the future of alliances looks quite bleak. I can see the allaince model being suitable only for trips where part or all of the journey is long-haul within the next few years.