The bigger a company gets, the less they need to worry about individual jobs, and more about quantity.

Yes they may still be great at what they do, and have all the tools but they cant spend a great deal of time on one job, while the large workload due to lots more marketig and advertising rolls in the door.

Smaller shops as mentioned might take on the job for other reasons as mentioned, personal interest, build skills, humanitarian reasons, need the cash, enjoyment etc.