Why exactly is it bad etiquette to ask to see a losing showdown hand?

[Assani Fisher]

Like many of the younger players around today, I started playing online before at a casino. And online, its very easy to click on the hand history box and see what cards your opponent mucked when he lost at showdown.

Now I have never asked to see a losing hand at showdown in person because I know that its considered bad etiquette. But my question is why? Poker is a game about acquiring information imo, and a player should be able to do anything within the rules in order to help him do this. Furthermore it just makes sense to me that both players SHOULD HAVE TO show their cards at a showdown.

Can anyone explain to me the basis behind the etiquette here?