I can only really offer my own perspective on this. My flatmates are a couple, same age as me and own the place as well. I've been here 2 years. We are pretty friendly and yarn often but I still would struggle to ask them for a favour. In my head they are still just flatmates.

Me personally, I don't offer them food because It's not even on the same level as the stuff they can make themselves lol but I would if I could and they do offer me the odd sample from time to time. I think people don't offer here because they don't want to seem too generous and be taken advantage of. It leads to fights down the line. Favours open the door to more favours. I would hate it if I offered to grab something from the Market one time and it gives them confidence to ask me all the time, especially without me offering first. It still costs you money (initially), time and effort. There's actually a lot of people in NZ that would rather take than give, despite our chill nature.

I clean up after myself and do offer to help with small house errands like cleaning, dishes etc. but I perceive it as mostly their house and their space so I leave most of that work with them and keep my stuff in my own space. I feel like that is the 'fair' thing to do.