I would never say “Dad and Mom,” or “small and big,” or “white and black,” for example. Why do we place things in this order?
Pepper and salt
Vinegar and salt
Ringo, George, Paul and John
Why do we do this?
To me it’s
Gin and Tonic
Salt and Vinegar
Salt and Pepper
Exactly my point. Why?
ETA: Fork and knife.
I note that all of the pairings you list have the one-syllable word first, and the multi-syllable word last. I have no idea if that’s why the preferred order of those couplets are in that order, but maybe there’s something to it.
Yes and no
Up or down
Front to back ETA, this one seems to work either way.
Cats and dogs (stretch)
Boys and girls
Ladies and gentlemen
Husband and wife
There really is no rhyme or reason. (another).
Fork and knife
dad and mom
ETA: That was for Kenobi
When I was small (around five), my parents tell me I used to make a game of reversing a lot of these - pepper and salt, chips and fish, and so on. This came to a head in a cafe by the beach when I asked the waitress where my “fork and knife” was and she thought I’d sworn at her.
No Spoons? 
Yeah, it’s clear that “multi-syllable last” isn’t the sole determinant, since there are lots of one-syllable-and-one-syllable couplets like those.
And, as per Beckdawrek’s list, there are at least some that disobey that theory entirely, like “husband and wife.”
“It’s nine o’clock on a Saturday
The regular crowd’s smokin’ chronic
There’s an old man sitting next to me
Makin’ love to his gin and tonic.”
yon & hither
the why, how, when, what, who and where of it
Yang and yin
Forth and back
Roll and rock
Blues and rythym
There must be a hundred of these!
One that is awkward is when you have a friend named Jim with whom you somewhile or muchwhile consort. To speak of your combined activities, you would begin with “Jim and I …”, but in common speech patterns, it tends to slide into almost “Jiminey”. So, you end up finding ways around that phrasing.
OP asks “why?” and just about every single response ignores that. Because Dopers.
It seems to be a combination of
putting the important/most popular thing first: Salt and pepper, Gin and tonic, John Paul George Ringo etc.
Things that are the de facto name of a genre, which may have more aspects to it but somebody defined a couple of them and it it in the order and it stuck. Rock and Roll, Country and Western etc.
Personal experience may differ: White and Black and Dad and Mum both sound fine to me.
This doesn’t cover everything though.
Fro and to. Yon and hither. You know, the whole caboodle and kit.
Shodan,
Regards
There is no good answer other than, “just 'cuz.”
In Japanese you say “white and black.”
If one ever talks to a Cajun for any length of time, you will probably recognize that, when referring to a quantity of things less than a dozen, but more than a half-dozen, the Cajun will often say “eight or seven” where I find it more common to phrase it “seven or eight”. As in, “Hey, Thibodaux, did you go that Fais do-do at Boudreaux’s Saturday? There musta been eight or seven pirogue boats out behind his place.”
Interesting you chose the numbers seven and eight. Old Testament writers used to use that to mean (and I quote Holy Scriptures here) “a shitload”. I have a couple of friends who do this, often prefacing it with “Verily”.
“Seven times has this happened, even eight…”
Here’s the poet who wrote Ecclesiastes:
“Cast your bread upon the waters, for after many days you will find it again.
Divide your portion among seven, or even eight, for you do not know what disaster may befall the land.
If the clouds are full, they will pour out rain upon the earth; whether a tree falls to the south or the north, in the place where it falls, there it will lie.”
Deep, maaaan…