"all but" is short for "all but < enumerated exceptions >". the speaker is just eliding the exceptions. people using it as an adjective/adverb phrase are just nodding at the whole thing, even though the exceptions wouldn't neatly fit grammatically. wouldn't be entirely surprised if it got smushed into a single new word over the years - albut or something.
I agree with "could care less"
also "all but" meaning "very nearly", I know there's probably some explanation but it still Grinds My Gears(TM)
"all but" is short for "all but < enumerated exceptions >". the speaker is just eliding the exceptions. people using it as an adjective/adverb phrase are just nodding at the whole thing, even though the exceptions wouldn't neatly fit grammatically. wouldn't be entirely surprised if it got smushed into a single new word over the years - albut or something.