Why is Jack a nickname for John?

I've never found a definitive answer to this.
Just as i have never been able to find a definitive answer for why William is shortened to Bill. Or Charles to Chuck or Peggy from Margaret. Or Ted from Edward. Or Harry from Henry. Or Sally from Sarah.

I [i]suspect[/i] there are many things at play here.
Some might be down to regional variations of the same name.

Some may be down to to Senior and Junior. Senior being called by his proper name, junior by the given epithet. (John-Snr. Jack-Jnr)

Some may be down to linguistic differences. How names sound from one dialect to a different one. given that at one time w were all in different tribes.

Some may be down to incorporation of one language to another following war loss. The conquering foreigner referring to people's names differently as a sign of re-naming the enslaved to show they no longer had even the independence of their given name !

I’ve never heard Jack used as being short for John. They’re both 4 letter 1 syllable words.

Ikr? Like my name for example. People call me Bo but my full name is Bozo.

I dont get why Jamie, Jim, Jimbo, Jimmy are short for James 😑. James is such a sexy name and those are NOT

...or Bill, for William, Bob, for Robert...

I see