People will still join for social reasons. They do it outside of prison, so there isn't much reason to discontinue the practice inside of prison.

Actually self-defense is one of the main reasons people join street gangs too; they live in violent neighborhoods, go to dangerous schools, etc. You obviously don't need a criminal enterprise to get social interaction. Even if some people join gangs just because they like being in a gang, it is a well-established fact that most of the people who join prison gangs do so because they feel it is necessary to protect themselves.

Solitary confinement has long been associated with negative psychological outcomes. Some would call it torture.

And I suppose being raped, terrorized, beaten, etc. are not forms of torture? Of course, the modern practice of solitary confinement is designed as a punishment, including not being given visitation, yard time, television, etc. That is nothing like what I propose, which would involve using visitation and work-details so prisoners get to socialize with non-criminals.