... it's more difficult for people in their 30's and older to make new friends. People by that age already have made their social circles. It's the one problem that is difficult to solve for those who leave the cult.
It is by design.
Watchtower keeps JWs isolated so that the only social life they have is with other JWs. The Organization plays on a very powerful human need, the need for socializing.
If JWs were allowed to make friends outside of the Watchtower organization leaving would be easy, but Watchtower uses isolation as a tool to keep people trapped inside.
(To me, this is a mark of a very high control cult religion.)
I think there are many JWs who stay in for social reasons only, although they might not want to say it out loud, and even though the friendships are very artificial. In a way I don't blame them. Some people can cope better living life without strong social connections, while others need it desperately.
But even if many stay in for social reasons, it still creates a weakened organization because their hearts are no longer in it, and usually neither is their money.