I've always thought it had to do with ergonomics. reaching for e.g. qwer or even 1234 from the rest position asdf would put more strain in the fingers without the slight shift that makes it more of of a side motion. compare the strain of reaching from f to 5 to that of reaching for the 6 with the same finger. if the keys were like a typematrix, the 5 would be where the 6 is. it doesn't feel comfortable to me.

now, that thinking doesn't seem to apply to the right hand, so maybe it is mostly legacy of mechanical typewriters after all

I guess better ergonomics could be achieved by positioning keys in curved lines, so that the rest position had all fingers equally rested, something like asdc instead of asdf, rather than horizontal lines of keys. but transitioning to such a different layout might be a pain for touch-typists. at this point, it probably makes more sense to leave us alone with our weird ways, and think of ways to improve the far more widespread thumb-typing