The #1 purpose of such wall warts is to provide safety by means of galvanic isolation (in use, any part you can touch on wall wart or device it powers, has no d

The #1 purpose of such wall warts is to provide safety by means of galvanic isolation (in use, any part you can touch on wall wart or device it powers, has no direct electrical connection to HV side). That's where a transformer comes in. When using a transformer, the voltage conversion comes 'free'. Modern electronics makes this smaller, lighter & using less metal (not more reliable, btw ;-) On the generation side (for example rooftop solar), it isn't a big deal to have 1, powerful, high-efficiency converter.