Interesting! I didn't realise this until just now when I did the exercise. I suppose the notion is similar to nouns like scissors or trousers in English. Are there any other examples of mass nouns that are found in the plural only in Ukrainian?
It's like fish or deer or moose in English. There are a few others in Ukrainian that i came across so far in Duolingo but i am drawing a blank as to which ones they were. I'm sure you'll encounter them.