Translation:There are forty-seven spiders there already.
The English sentence needs a second "there" because of tam. A possible alternative that eliminates the (apparent) redundancy might be "Forty-seven spiders are already there." I will be happy to add that, if the Czech natives on the team feel that would not be a different sentence in Czech.
What do you mean why not? It is accepted. Please ask complete questions, we often just remove these unclear "Why not X".
The Czech sentences are primary here and are selected so that they show the possible variability. They do not have to be neutral or most natural. If something wasn't accepted, use the report button.