Because it actually makes the game a lot more strategic in some cases. Let's stay with your example of the random Deathwish. If we look at Rotfiend, he is able to deal 4 damage to a random enemy. Basically he does ranrdomly deal any damage between 1 and 4 depending on the state of the board. But if the player removes all 1 and 2 point units from the opponents side before using rotfiend, he will deal at least 3 points of damage.
Furthermore having such random effects is a way to balance certain effects, because being able to direct damage is always a lot stronger than random damage (except if the enemy has immun units), because you can focus the key units. So having an engine that deals 1 damage at random, actually makes it strength really calcuatable, because that 1 point of damage will be only 1 point of damage and nothing more on average, while 1 point of directed damge can allow players to focus valuable targets and therefore allows them to shut enemies down.