Hmmm. I think this is probably true for a lot of people, but not for me!
While I don't have direct, conscious control of when exactly I fall asleep, in my experience, it's felt like I have much more control over that than when I get up!
My father actually suggested setting a dedicated wake-up time in order to force bedtime a long time ago. I have tried it now and then--including Steve Pavlina's exercise of "practicing" getting up when the alarm goes off over and over. Nope.
When I wake up feeling like I haven't gotten nearly enough sleep, I often have a strong fear that if I don't go back to sleep, the rest of the day is going to be crap. Sometimes that is a justified fear. I am not rational enough when I wake up to prioritize an long term goal over the anticipated pain.
Urgent things and some social obligations can "force" me to get up earlier, but unless I also make a systematic effort to sleep earlier, I just end up chronically sleep deprived. This is partly because being sleep deprived greatly diminishes my ability to make good decisions, like making arrangements to go to sleep earlier. So earlier wake up does not force earlier sleep for me.
Anyway, all this is only ancillary to the discussion--I have trouble getting up, but unlike @bee I don't have trouble getting enough sleep if left alone.