hey buddy, this is how card games have worked since the beginning.
the simplest explanation is that those games started as physical games and there is no good way to keep track of such things in your hand since usually you would use some type of a counter(dice, pebble, or a count) to denote the strenght/defense of a card when it is on the battlefield and you can't hold counters in your hand.
the second thing is that in most games you are some type of a wizard or a caster and the cards themselves are your spells so it is not really the card that goes on the battlefield but the result of the card being used.
the gravestone situation is kind a the same as turning something in a gravestone is basically recasting it. but because of that you can have cards that turn into gravestone and cards that make a creature invulnerable. and you can use this mechanic too, for instance if your opponent gives a creature -1/-1 you can gravestone it, and remove the -1/-1 🙂
