I think they're taking you for granted .You seem like a nice person, your heart is in the right place . I wouldn't dream of going to someones house on a regular basis,for whatever reason , without at least offering to help clear up afterwards or at least bring something for people to share,if it was only a packet of biscuits .
If it's getting you down ,speak up and tell them how you feel .Don't be a shrinking violet :)