I think it’s just a reflection of the fact that daughters/daughters in law generally put more effort and energy into managing and maintaining all the familial relationships, and that this usually results in a natural bias (conscious or otherwise) towards her own original family unit.
I wouldn’t worry too much re the holiday situation. Myself and my DH have been together 15.5 years and in that time have only been on one holiday abroad with his Mum, Stepdad, younger Bro, fiance, their kiddies, and 2 trips with his Dad and Dad’s partner. Over the same period, and excluding the few Covid years with restricted travel, we’ve holidayed abroad with my parents every year and also had a few UK breaks together. We’ve also had 4–5 wider family holidays abroad with my parents, brothers, SILs, niblings.
Some of that is influenced by factors like his Mum and Stepdad being extreme creatures of habit: same two weeks, spent in the same Turkish resort, at the same hotel, eating in the same restaurants EVERY SINGLE YEAR - they love it, but it’s definitely not our idea of the ideal holiday. Plus them preferring to eat a very early dinner and go to bed early, and whilst we are far from crazy party people, we like having a few drinks and staying up slightly when we are kicking back on our holiday.
But honestly the main factor just comes down to basic consideration and planning. DH’s family tend to go ahead and book their trips very much for themselves, and wouldn’t even think to ask us about our plans and if we’d be potentially interested in going along. Whereas between me and my parents, we will chat about our potential mutual holiday plans for the upcoming year well before anything has been arranged.
I’m also increasingly conscious that my parents aren’t getting any younger and are suffering with more and more health issues, and with no guarantees of how much time we might have left, I want to enjoy every opportunity to spend quality time with them whilst I still can.
Although I’d say my DH is generally pretty good and considerate with regarding his family, how often he sees them etc, I’m definitely closer to, and make more of an effort with, mine. I think it’s only natural that the result is us spending more of our time in their company and, by that measure, “choosing” them over his.
Try not to dwell on your son seemingly ’choosing’ his GF’s family over yours. Instead, be glad that he’s got a great set of “in-laws” who like him/he likes and are already happily treating him as a part of their family. Try to invest your energy in ensuring he and his GF feel as welcomed, loved and appreciated when in your company as possible. That way when they do choose to spend time with you, you all have the most enjoyable time in each other’s company and you’ll be laying the foundations of a relationship that is solid and mutually supportive/appreciative - should their partnership ultimately progress to marriage and/or grandchildren, the benefits of that approach will quickly become apparent.