one would think people would be all over this game now

because very hard is not the fastest difficulty

honestly it would barely make a difference in the run because the vast majority of damage taken comes from falling, it would only be more annoying to run in places where you can get attacked

but then why does dishonored 1 have very hard speed runs?

You don't need a leaderboard to speedrun a specific category. If there are multiple runs of a category, then mods will maybe add them to this page.

Creating a leaderboard for a category nobody runs is pointless.

Basically very hard means guards to more damage, you get less health/mana from potions and you get it slower. In practice this means taking damage is less forgiving, but that really doesn't matter since you don't take much anyway, and that leaves us with a run that has less tricks in it, due to having less mana, which means more downtime of running and arguably easier to compete in.

tl;dr: Very Hard makes the game less interesting to run.