Why does the dealer in blackjack receive only one face-up card, not two like the players?
In Black Jack, you play against the dealer, not the other players, and one of the main mechanics is deciding whether to hit or not. This decision becomes trivial and meaningless if you could know exactly what the dealer has. You hit if you have less than them; stand if you have more than them (if they have 17 or more so that they won’t hit). Hiding one dealer card is necessary to make it so that there’s a non-trivial choice to make.