Why student attendance is not considered a fact in data warehousing?
Blame Kimball for the confusing term. Attendance is a fact (as in, "a thing that is known or proved to be true"). However, it is an event, and as such it does not have any extra information apart from references to the corresponding dimension keys. It probably should be called "measureless fact". 

A sale, in addition to the dimension references, has measures: quantity and price.