>I imagine that in places where electricity is scarce
I think that's what most people in the west imagine and that's why this is a gimmick that resonates with well-meaning westerners. Besides the fact that developing world is a diverse place in all aspects (so the the stereotype you're imaging is wrong for the vast majority of the developing world), communication is such an ingrained human need, that communication devices like smart phones are everywhere.
But none of that matters. These panels just do not work well - or at least as well as you can expect given the surface area and whether or not there's enough sunlight. That's why nobody bothers with building them into devices. Besides you can buy little external solar chargers for real cheap. And the external ones are better than the ones built into the devices because you can angle them, and they aren't a failure point you need to worry about (that is, if they die, your device doesn't need to be replaced), and aren't limited to device size. Also, I don't know why you would want your electronics exposed to direct sunlight... Heat is not good for electronics.
Again, I support low-power, energy efficient devices. There is a very real use case for them, and not just for the developing world. But these gimmicks, like building in solar panels into the device, are dumb.