Why do we use "is" instead of "has been" in the below example?
Lisa is in Germany. She (is/has been) there on a business trip.
Both are possible, in suitable contexts.
Without any particular context, has been there will be likely to interpreted in the special meaning of have/had been meaning "gone somewhere and returned", and so it will not fit with the first sentence.
But in answer to a question that sets the focus on recent time, like "What have your sisters been doing the last couple of weeks?", you version with has been is fine.