Generally, they don’t have the depth. Go has a huge game tree, so most bots read fairly shallow variations. Humans can easily look at a ladder and say “I’m going to read this out 30 moves and see where it ends up.” Without ladder-specific code, bots won’t do that. Plus, if you don’t know that some ladders are dead, ladders all look like broken ladders, which are great for the player in the ladder.
Some (but not all!) good bots have ladder-specific code to help them get around this. Those bots are not kyus.