I have 2 kids, 8 and 5, I'm only 33 so had kids before most of my friends, a few have had kids in the last 2 or so years.
This year I have been to the birthday parties of 3 of my friends kids, 2 first birthdays, one 2nd. The last party was just this weekend. All 3 had a similar format, all the parents friends (regardless of if they have their own kids), random start time (1/2 in the afternoon), no real end time. The adults drink, the kids run around, no activities, at some point they do cake and everyone gets a cuddle with the birthday kid but its really not about them. In august one started at 2pm and people were still drinking in the garden at 11pm!! There are always people there who have never met the kid before and haven't seen the parents since the baby shower, so it is really not about the kid!
For my kids first/second birthday we had family and close friends round. Only started parties when they had friends to invite.
AIBU to think it is really odd to spend hours getting drunk at a kids party with adults they have never met and have no kids of their own? When did this become a thing? Is it a thing in your social circle?