Can someone explain me USA obsession with prom and similar school rituals?

I think it's important to remember that the USA isn't a single culture. Things vary dramatically even within a single state to say nothing of differences between states.

In some areas prom is very important. In others, not so much.

Only one of my three kids went to prom (Eastern PA).

Prom in my high school was a relatively big deal. You rented a tux or bought a dress. Some people would rent a limo. The prom was held in some kind of banquet hall with a fairly fancy meal. There'd be a DJ and dancing.

My wife was one year behind me in high school, and we attended FOUR proms (my junior prom, then the next year her junior prom and my senior prom, then the next year I came back for her senior prom).

I think for most people it's just an opportunity to get dressed up, have a good meal, and dance. If you're already dating someone, it obviously has more significance, but I had plenty of friends who just took another friend as a date for the prom and others who didn't go with anyone. However, there was a lot of pressure to be a "couple", even if you weren't actually romantically involved with your "date".

Typically the parents take pictures of the kids in their dresses and tuxedos. From the parents' point of view, it's a moment to sort of take note of how your kids are maturing and think about what the future holds for them. Lots of thinking about how old you are ;-)

Often there's an after party that goes on late into the morning, and for many kids the after party is more important than the prom.

I think social media has had an effect on what prom is, but it also has the effect of distorting what it is to people who only experience it remotely. When you're seeing the crazy YouTube videos and Instagram posts, you're not seeing what prom is. You're seeing a snapshot of what those particular proms are.