I'm nodding along with Sal, Suz, Mary, Jenn, Ledonna and Helena.
Jenni, I think there are many factors at play, which have come through in the comments already. Lots of interesting thoughts!
Generational differences, cultural differences, context, and a world that is more casual than it ever was makes its impact. It was cultural for my Euro Dad and Eurasian Mother to dress up for occasions, and expect their kids to do the same. Their family members had the same set of dressy and "respectful" values. You even dressed up to fly on a plane back in the day.
Conversely, my husband's South African family is very, very casual because its part of the culture. You could have Christmas lunch in shorts, tee and flip flops. But in my home with my parents, it was your Sunday best. I don't think my South African family is being disrespectful. The dress code thing is simply not on their radar. They are happy to see me dressed up because they don't know me any other way. And everybody is very accepting and non-judgmental - for lack of a better way of describing it.
My 85 year old Dutch Aunt will turn her nose up at my fashion sneakers when I visit for tea and cake in the Netherlands. And I am a dressy person wearing immaculate white sneakers! I know better now, NOT to wear sneakers when I visit
At my Mother's funeral in Cape Town 21 years ago - all dress codes and colours were represented. Formal suits and dresses, jeans, and shorts and t-shirts. Some people's very best was a new pair of jeans and lace-up shoes, whereas others just didn't care and rocked up in whatever. The former is respectful despite the lack of formal attire because they made the effort, and looked clean, neat and presentable.
Times are changing, and my own dress codes have relaxed over the years too. Hubs Greg is much more casual than me, and likes that I dress up! He says that as long as I dress up, he gets away with looking more casual