Book question

Has anyone else read "Never Let Me Go" by Kazuo Ishiguo? I loved the philosophical issues raised by this book and have been thinking about it ever since I finished it, but despite putting a lot of thought into it I still have a question that I can't answer.

(WARNING! MAJOR SPOILERS BELOW!)

What was the importance of the carers' driving such long miles? It was referred to so often that there has to be something I'm missing. Kathy as narrator often mentioned that she had driven from her client in North Wales down to Dover while caring for Ruth, for example. Why was it important that we see the carers doing the miles and being exhausted? Are we to infer that they're simply too tired to think about how their lives might have been different?

Or was it to show how thoroughly indoctrinated they were into their purpose in life? That would also explain why Kathy and Tommy didn't simply drive off into the distance after their visit to Madame, instead never even questioning that Tommy would go back to the Centre and make his fourth donation.

So. If you have read it, and have some thoughts about this question, I'd love to hear them.