It is simple, the training company's have sold the idea to the airlines that it is better to have someone who has done the integrated course, if you are part of one of the airline "attached" courses the airline gets tax breaks that results in your training being almost free to the airline and you the pilot being tied to them for a number of years.
The advantage to the training company they can provide a linked up modular course and charge you the pilot a lot more money for a "dressed up " product, I suspect a large slice of selling a dream and marketing to the money providors (be it the bank or Mummy & Daddy).
If all of you guys went "modular" You couls be £15,000-£20,000 better off at the end of the course with the same licence and it would force the airlines to shop around for pilots. Who knows they might even start paying for your type ratings. (would it not be refreshing to see the threads about training bonds return?)