Power supplies are one of the most common failures in electronics, so having an external one means they are easily replaced, plus there are plenty of quality 3rd party ones at a fraction of the price of a brand name one. (Manufactures don’t make their own (Or at least not often) they just buy them in with their sticker on)
On the flip side they are a pain, but for gigging they are probably ideal, as if they fail you can pop down to an electrical store and pick one up, (Or keep a spare with you) thus your instrument will be down for far less time than having to take the instrument in to a service centre. (Time is money)

Bill