I know you’ve heard this one before. And I concede right now that there’s no logical reason why “already” should be spelled as one word with a single “l”, while “all right” is two words and a double “l”.
Language isn’t consistent, okay? It’s in a constant state of change, and not all the bits of it change at the same rate. At some time in the future, “all right” may well have finished mutating into the single-word, single-“l” form — but that time is not now.
There. I’ve got that off my chest, so it’s back to the word mines.