Could it be a matter of culture in the language? Coming from Canada, one would never use (at least I've never heard in my lifetime) sit "in" bed. We would always use the expression sit "on".
For examples: a) "There is a monkey sitting on the bed." b) "Why don't you sit on the bed?"
Now, if you were to be talking about occupying the bed using "in" as suggested in this lesson, we would use the verb "to lay", rather than "to sit".
For example: a) I'm reading a book while laying in bed. b) Should we lay in bed?
Remember, the English language is quite diverse across the world and Duolingo is only doing it's best to bring it all together. Hopefully this specific lesson will be withheld from Tree 2.0.
Det finns would imply a sense of permanence to the monkey in the bed. As I suspect that the monkey is not a permanent fixture on the bed, "det finns" sounds a little odd. Sitter/ligger/står are all used to describe existence/location of something so this sentence is quite natural particularly given that the English translation specifies that the monkey is sitting.