Inertial charging still requires that you put the effort in, the trick is that with watches it's very little effort – equivalent to winding the watch every few days – so you don't feel it.

If you have ever used a hand-crank torch or radio, or something else that converts effort into power, you'll know that it takes a fair bit of effort, and that's for low power devices. Inertial charging to recoup, say, 5k mAh to charge a smart phone for a day, would therefore require a lot of effort. This would be spread out over a day, but might feel like, for example, carrying a heavy backpack the whole day.