Y do teachers give us so much cw?

Unfortunately its not upto teachers to decide how much coursework you get, its all down to the people who decide the curriculum and the exam boards to decide how much coursework you get. Also, the more you get, the more they know the mark you receive is deserved, i.e if you just had one exam and it fell on a bad day you might not get the mark you receive but an average of a number of pieces of work is more fair.