Outside of Christmas the only real tradition my family has is mother's day lunch but partners aren't invited to that one. Sometimes my mum invites me for dinner on a short notice but that's always when he's working so he doesn't get the chance to join. In his family we only occasionally meet up with some relatives one on one, they don't have real family occasions ever. I have tried to invite family over and start new traditions but it seems no one is really interested in that. I feel like I'm trying really hard but there's just a mutual disinterest at this point that seems impossible to get over.
I hear you on this. Family traditions can vary widely. In my family, we celebrate just about everything but DH's family just doesn't have real family occasions now. Back when we were first married, they did the typical holidays of Christmas, Easter and Thanksgiving but things like birthdays, graduations, etc. was done more one on one. Birthday and graduation parties were just Mom, Dad, one set of grandparents, DH and his sister, DH never had the bigger family celebrations that I had. This continued into the next generation with niece and nephew as well, they never had big birthday or graduation parties, in fact they were treated like they were non-events, really. Even DH and I, who were their only aunt and uncle, did not attend a graduation party until the nephew graduated college and that was only because of his sister. Niece tried to change things after she had her own kids with some new traditions in the family but quit after a few years. Earlier in our marriage, I offered to host the holidays, but no one wanted that, there were all sorts of reasons why we couldn't do that. After a couple of tries, DH told me to not offer again, it was obvious they were not interested in doing anything different.
I hate it when people do their absolute best to avoid contact with one's extended family or social network and then bitch about "not being included". To me, not wanting to interact with a partner's family is a gigantic red flag. Sometimes it means that there's a lot of family dysfunction, but if there are more than about a dozen people you can generally find one or two that click well enough to justify at least a social media friendship. When a partner, or prospective partner, fails to create a relationship with even one member of one's family or extended network, that's an indicator that they do not wish to interact. It's reasonable to do that, if a group of people is toxic enough, but the person declining to form a relationship should at least own their decision to maintain the distance.
And IME it's not just about partners but also partner's family. DH is considered a part of our family, he has made the effort to get to know them and gets on pretty well. But his family is a whole different story, namely MIL and FIL complain that they "don't know our family" and "they are not included". This is not because of a lack of effort on my family's end.
In our family, there generally is an effort to get to know a partner's family. Usually a partner's parents are invited to major events like weddings. We try to get together one on one. We would attend their funerals. Partner's family occasionally were at holidays depending on what was happening that day with the other side of that person's family. As a result we got to know the other, aka in-law, side of a family member outside of mutual family events like a kid's birthday or graduation. MIL and FIL were not treated differently, the difference was that they rarely attended, and had a tendency to back out after accepting an invitation. My folks invited MIL and FIL to my sisters' weddings, they turned it down. My folks made an effort to meet with them either at a restaurant or host them at their home. It happened a couple of times but eventually it stopped because MIL would accept then back out at the last minute. The last time my folks invited them over to their house, MIL had one of her panics over "the weather" and backed a couple of hours before they were supposed to arrive. Highly annoyed after cooking all day, Mom threw in the towel, I remember getting the phone call that day. My parents attended funerals in DH's family, but MIL / FIL mustered only one attendance in 35 years, that to my grandmother's funeral 14 years ago. They backed out last minute on coming to my Mom's memorial service which infuriated DH but I was not surprised. They didn't even bother with my Dad's.
So yes, they don't know my family and it's all on them. But of course they don't own that, it's all excuses like "we see our family more than them" and we are playing "favorites" or some other reason that is always someone else's fault but it's never theirs. For DH and me, we know they are not just interested, they did not want to put in the effort and believe me, my folks tried to to make things as effortless as possible in order to get to know them and include them in things. They just didn't want to bother so this is the end result.