Because walking to university (40 mins each way for me) isn't particularly an efficient use of my time for a one hour lecture that becomes available as an audio/video recording 5 minutes after the lecture finishes. Which is why I missed one of my 16 hours of contact time this week.
They have to submit attendance registers to admin at UW for all compulsory lectures. That said in my third year, I've only got one module with a compulsory lecture every Wednesday and then the others have been as and when they need to call a session.
Oh wow... our lectures are only monitored if the lecturer specifically hands out a register - happened once in all of my lectures this semester and last, and he even gave us pre-warning...!
Next week is week 5 for me (all exams are in the summer) I've missed none- that said I have 3 hours of lectures a week and 15+ hours of my research project to do too
In four years I've skipped two lectures - one I was vomiting and the other I was at a flat viewing. Sure, I haven't always given my full attention, but I need to get my money's worth