Yamaha uses the external power supply because its cheaper for them to do so. Many countries don't have stringent power requirements like the US does and don't require UL listing. If Yamaha built an internal switching power supply that could switch various voltages for all countries, it would increase their manufacturing cost. With the PSR series you're essentially buying a powered toy so Yamaha doesn't care if they give you a cheap generic power supply. You want an internal supply, don't buy cheap plastic toys, buy a professional grade instrument.