The reason people don’t get much done on a workday is explained with a diving analogy.
If you want to dive deep, you don’t want to have the feeling that you have to get back up to the surface anytime soon. However this is exactly what happens at work. if you need to do a task in which you need to dive deep and really get into it. You can’t dive deep if you have the feeling you have to get back to the surface soon. You don’t want to have the feeling that you need to get back to the surface anytime soon for a meeting or interruption. This is what Paul Graham describes in Maker’s vs Managers time.