Do you think that sometimes people just skive?

I've always had almost pefect attendance at whatever I've done and university is no different, noticing though that when the going gets tough or that there's lots of works to be done, many of the people I'm studying with will take days or even the whole week off and then make up some excuse when they come back about being ill, which I'd believe if I hadn't seem them out and about or asking me if I want to go out somewhere, why do people do this? really frustrates some of my lecturers as they need people to attend our cles due to the course we're doing xxxx