The problem is that time isn't something you can chop up neatly.
For ease of use, let's say that a story point represents 8 hours of actual work. That gives us 168/8 points a week, or 21 points. Of course, we lose 7 of those to sleep, 4 to the weekend and another 5 to the evenings during the week, leaving us with 5 points per week. You probably lose a point to lunch, coffee, bathroom breaks and mingling with coworkers and another point to meetings and interruptions.
That leaves you with 3 points out of 21 potential points in a week, and you aren't getting 3/5 done per day. Using story points instead of hours tries to get people to treat the sprint as a unit instead of time as a unit, since time can be split up to a very fine degree. Just because you work 40 hours a week doesn't mean you can complete 20 2 hour tasks.
For ease of use, let's say that a story point represents 8 hours of actual work. That gives us 168/8 points a week, or 21 points. Of course, we lose 7 of those to sleep, 4 to the weekend and another 5 to the evenings during the week, leaving us with 5 points per week. You probably lose a point to lunch, coffee, bathroom breaks and mingling with coworkers and another point to meetings and interruptions.
That leaves you with 3 points out of 21 potential points in a week, and you aren't getting 3/5 done per day. Using story points instead of hours tries to get people to treat the sprint as a unit instead of time as a unit, since time can be split up to a very fine degree. Just because you work 40 hours a week doesn't mean you can complete 20 2 hour tasks.