David:
Traditional dado sets are available in 6" versions - FreudSD206. I agree that it is seldom we need a dado much deeper than 3/8" so that extra metal is not doing much.
For those who make a lot of dados or have a larger budget than I do the Freud Dial-a-Width is an interesting option.
And those lovely thin brass spacers that come with traditional dado sets sliced my fingers more than once before I replaced them with magnetic spacers.
Bill