Me, before I was married, and lots of younger couples.
Heck, I moved in with my now wife only about 90 days after we started dating, without even talking about it. We just got together for dinner and a movie at her place one Friday night, had a great time until very late into the night, stayed over, spent the whole next day together, had a great time until we were really tired so I stayed again, etc... I just went over one night and never left. I still had my place for a several more months, but I was never there, and eventually I got rid of it.
It wasn't long until we opened a joint account, probably within the first month of living together, just for stuff like food, and then rent, and eventually growing into our family budget. We both kept our own accounts as well, and we still do to this day - That way I don't care if she wants to spend a fortune on Christmas presents and clothes for our kids, and she doesn't care if I spend my money flying airplanes. Our respective paychecks are deposited into our own accounts, we each contribute proportionally to the joint account each month, and we never have to disagree about money. It may not be the old way, but it worked so well for us that I can heartily recommend it.
The other thing is that a lot of younger couples are forgoing marriage entirely because weddings are too expensive. Same with kids. And some purposely have the kids before the wedding. Finances are very different and much more difficult for younger generations, so this sort of thing is becoming a lot more common. Probably not that big of an issue if they're buying FSD and $8,000 engagement rings though.