Well there is if you go far enough. It's just the question when did they split off from each other. However there is no question that Baltic and Slavic are more closely related to each other than any other non extinct Indo-European languages.
The fact they they are the closest surviving relatives on it own doesn't mean it makes sense to group them together (i.e. Italo-Celtic is also a theorized subgroup in a similar way but nobody is disputing that Celtic and Italic languages evolved into distinct groups).
Then there is a huge amount of missing links and unknown unknowns. e.g. Thracian and Dacian probably were also pretty close to Baltic or Slavic (maybe even closer to Baltic than Slavic is but we don't know enough about them to make any conclusive claims at all... but we at least know these languages existed)
The fact they they are the closest surviving relatives on it own doesn't mean it makes sense to group them together (i.e. Italo-Celtic is also a theorized subgroup in a similar way but nobody is disputing that Celtic and Italic languages evolved into distinct groups).
Then there is a huge amount of missing links and unknown unknowns. e.g. Thracian and Dacian probably were also pretty close to Baltic or Slavic (maybe even closer to Baltic than Slavic is but we don't know enough about them to make any conclusive claims at all... but we at least know these languages existed)