Because there isn't any archaic form for health. Unless you would like to adopt it just now....maybe we can refer to it from here on out as Healtha.
Which means that you, Llamaskull, will go down in history as creating an archaic form of a modern term.
Gratz.
"Magicka" is a made- up word. It is not an archaic form of a modern word.
A- Because of all those words, only magicka sounds cool.
B- Because of these 3 stats, only magicka does not have a common everyday word which it makes sense to refer to it.
Magic is a common everyday word which makes sense to refer to it by.
I think it's all about coolness and there's no defensible, factual logic behind it. I'm just going with the OP in saying that it's a little silly. Not a big deal, really.