I guess that will vary from person to person. Multitasking is definitely a big reason, you can do everything you want while you talk on the phone, meanwhile with video chat you have to be in front of the camera. Also, some people can be uncomfortable when talking using a videocam, in the sense that they can be in pajamas at home, in underwear, or doing those little things everyone does at home, and it's embarrassing for them to let others see them like that.
I suppose it's a matter of asking your girlfriends why they don't like it. Personally, I don't have a webcam, and never felt inclined to buy one for videochat, maybe because I don't have any particular reason about having or not having one.