Why the fuck does it matter how I hold my forkor what spoon I eat soup with? Why does it matter where my ellbows are?

Really depends on what part of it. There are things like offering your bus seat to someone who needs it, or waiting for people to exit before you enter. Those indeed make sense.

And then there’s what the other commenters pointed out, arbitrary rules about what cutlery to use and in which hand and such.

Considering every culture has completely different etiquette, I’d argue otherwise. We’re talking drinking from bowls vs talking during a meal style stuff. I’ll hold my fork with the right hand and knife in left, despite being right handed and no etiquette freak can stop me!

Except etiquette extends beyond just “common social etiquette”. Using a knife and fork the “correct way” is etiquette. Eating soup by scooping the spoon away from oneself is etiquette. Placing your cutlery the correct way on the dish when you’re finished is etiquette and varies wildly by country. These are just examples of dining etiquette, there’s much more. Its all bullshit and I agree it should boil down to being respectful and mindful, but depending who raised you it may happen that you get reprimanded and punished for not following very arbitrary rules.

One of the things my parents did understand correctly as “new money” is that a significant portion of piddling etiquette rules about what color to wear at what times of the year and which fork goes on the left were largely ways for the bourgeoisie to attempt to maintain their advanced standing against the increases in (the potential for) equality that capitalism initially brought about. Unfortunately my parents are also a very “well we got ours so everyone else must be lazy” type of people who think that’s as good as equality can or should get.