I have a hard time fitting in

I have been at my current role for a little over a year now. I am the most junior in the admin team at my current company. I feel completely isolated and ostracized by the team. No one speaks to me unless they need me to do something (Order their lunch, unclog the sink, watch the phones while they eat lunch together in the conference room right next to me). I understand that while making connections is a two way street, when I have tried to reach out to them to hang out after work, they have respectfully turned me down. I later found out that they all went to happy hour together instead. I have walked into the kitchen multiple times in the mornings and all of then would disperse and walk out gradually. I have done nothing but everything they have asked me to do to show that I am a hard worker and they hired the right person for this administrative role but that has gotten me into a position where they think I will do everything they tell me to. I find myself very isolated at my desk everyday in a pin-drop quiet office environment. I know I will be leaving this role soon, and I love my supervisors who I support. How do I tell them the real reason that I am leaving is because my team is very much a huge clique and that they don't speak to me outside of handing off their work to me.