1. The dynamo would take up too much room
  2. Pinching a tiny metal shaft is ergonomically awful
  3. have you seen the tiny wind-up flashlights?  they're huge, and they require quite a lot of winding, and that's just to power an LED for a few minutes.
  4. groundbreaking features like a wind-up piece of personal electronics, are *flagship* features.  If this thing had a 1 week battery, or was windup, or whatever, that would be a *very high visibility* part of the ad campaign.