How? Failure to launch (not necessarily a reference to that Matthew McConaughey movie)!
Almost every teacher I have worked with has thoughtfully engaged in the instructional planning process. They have thoughtfully selected the lesson they deliver to students for some meaningful, often instructional purpose. Most teachers are even excited to share those lessons with their students. That tends to be a constant -- teachers plan and teachers believe in what they do with students.
So why does it so often happen that students (especially our older students) tend to disengage and find little relevance in these well thought out lessons so quickly and so consistently?