Mostly because all the ones I'm aware of have stuff that will just fuck up your life as a solo person, and you can't sustain enough damage fast enough vs. incoming to make it work.
For instance, if you are fighting something that does biting plague every round... How the fuck do you deal with that solo?
Or smothering (unblockable?) for -str -con regularly... Or poison + strip + entwine + choke... or just shaman/invoker spec'd mobs with 10x your HP.
That said, a team of 2 can make progress under the right circumstances. Maybe a particularly well played undead necro could do something, but I don't think it's possible.
For reference, I've had more luck solo'ing Hell to the 4-5 circle than dragon lairs... Especially since there usually comes a point where you literally cannot flee (in dragon lairs) or you are totally boned, which makes soloing pretty much a non-starter.