That makes sense.

Lithium batteries (as found in phones, cameras etc) generally output between 3.7 and 4.3 volts, depending on the current charge level. As you use the battery the voltage goes down. And yet you charge the vast majority of devices via 5v USB.

I suspect that the direct voltage from most of the batteries is fairly useless (because it's vsriable) and so anything using lithium would need onload voltage regulation to get something a bit more constant.