Who are the “people” that you are referring to?

This makes total sense to me. There is no monolithic “culture”— there are multiple related cultures, differing little in essence but differing greatly in the details. And each individual is usually only partially ignorant anyway.

Culture changes, too, and asymmetrically. So the “done thing” may be done be very few anymore.

I feel like there was a brief period when middle class came to existence and started mimicking customs of the upper class, which were very complicated because the upper class was mostly bored and had invented this shit to kill the time. Then two things happened:

1. Upper class stopped being formal because formality stopped being a signal of upper class.

2. Middle class stopped having social gatherings in general.

So, like, "it is a part of the culture" in the same sense as traditional outfits are a part of the culture - most people have very vague awareness, nobody really cares.