It is a very semantic/grammatical reason. In English, "they" is a personal pronoun, and "those" is a demonstrative pronoun. In German, "Sie" is the personal pronoun (mandatory capital here), and "Das" is the demonstrative pronoun. That difference between strictly personal and demonstrative pronouns is what I think Duolingo is trying to "teach" with this example.
Having said that, I've never heard that kind of use of the pronoun. In fact, I wasn't tought that "Das" was a demonstrative plural pronoun. To say something like "Those are my cats" I would say "Diese sind meine Katzen", which is also not a direct translation. But commonly, for the same idea I would say "Sie sind meine Katzen" which would translate to what you put (They are my cats) and it is a more natural common way to express the idea.
Saying "They are my cats" is very different than "Those are my cats". Ex 1: Who's cats are these? - They are my cats.-or- Those are my cats. Both work. Ex 2: Which cats are yours? - Those are my cats. (Those over there, not these ones here). -or- They are ALL my cats. Simply saying "They are my cats" will not answer the question.
Why wouldn't you say "those"? If there were two groups of cats and you were specifying which group was yours, "those" would be appropriate.
Why isn't it 'Die sind meine Katzen' instead of 'Das ...'? Katze is feminine and on top of that Katzen is plural, so shouldn't the pronoun be 'Die' instead of 'Das'? Thanks for any clarification :)
I didn't really understand the explanations for demonstrative pronouns. What is the difference when using Das/Der/Die for This/Those/That. Are they all interchangeable with each other?