The examples you give are of those people who have asked you for your information to see you later and keep in touch. Maybe T's, do not personally want to be that close to the people they work with; it's a very intense, emotional job and it's like any other person not working outside their jobs? Personal trainers aren't training when they are with you outside of sessions but therapists would still be intensely being interested and they already have what friends they have chosen, outside their work, they aren't looking for friends among their clients.