My (future ex-)wife is the same. She says she loves her 3 children more than anything, and she misses them when she's abroad for work, but still, she chooses not to be with them.
Either they love their work more than anything, but don't want to admit it, or they love their children but don't feel the need to be with them as much as any other person.
Maybe it was a mistake for such a person to have children.
Sorry if I sound too harsh, but being recently in the same situation, but on the other end, is pretty rough :(.
People are different. Lots of people get lengthy deployments on oil rigs, in the armed forces, or even high pressure international sales gigs - I don't think these folks love their kids any less than me.
I love my wife and kids loads, but I also love being away from them for brief periods of time (only up to a week in my job). It makes it so much better when I return. We are all wired up differently emotionally - absence truly does make the heart grow fonder in my case.