Why do people abbreviate names to a name that is nothing like the original name

Question:

Why do people abbreviate names to a name that is nothing like the original name i.e Willam to Bill?

Also
Richard - Dick
Robert - Bob
Elizabeth - Betty
Edward - Teddy
Helen - Nelly

Where did this idea come from? and why are the names notning alike?

Best Answer - Chosen by Asker: I can add

Sarah - Sally
Margaret - Peggy as well as Maggie
I' ve seen women named Mary called Mamie
and Delilah called Dolly.

It might be the English language which is a mongrel language. England being out in the middle of the Atlantic has been invaded by many over the centuries.

First were the Celts. Then the Romans came and brought Latin. Then the Germanic Anglo, Saxons and Jutes were next. Then the Danes took over.
After that it was those tall dark French speaking Vikings called Normans.

That is why in English you frequently have several words to mean the same things, one word can mean something entirely different according to how it is used. Words take on different shades of meaning and double meanings eventually become a 2nd meaning. Gay when I was growing up just meant happy.

There is absolutely no true phonic system in Engish. So some of these nicknames might have been a variation in older languages.