Why is old ben is not a rebel?

Old Ben is not a jedi; he says so in Episode IV. He is sought by the rebels as their only hope and is pivotal in the destruction of the first Death Star and so could be regarded as a rebel in the same way he is regarded as a jedi for training Luke as a jedi despite there being no jedi order.
As for Clone Wars Chewbacca, he can't belong to a group that is yet to exist. Likewise, it doesn't make sense for him to be classified as a scoundrel as he wasn't such until later in the timeline. It would make more sense to have him without an affiliation but give him some jedi and clone synergies. This would not only fit the character better in terms of lore but also be very helpful to beginning players who start with all of these and gain little benefit with a lightside scoundrel.