Like the phrase "peeping tom" is used to refer to a creep trying to see somebody else using the bathroom. What on Earth did this Tom character do to become the definition of such a creep?
Why are women who do not have traditionally feminine traits sometimes called tomboys? Why Tom?
And why do people say "Bob's your uncle"? Was this Bob a super helpful and reliable guy, so much so that his name is used for the definition of something being finished and done?
And the playing card representing an aristocratic / upper class man is called Jack, who is this Jack? Why do people say a "jack of all trades", was this Jack character incredibly talented? Or is it related to the playing card, where they are good at everything but not the best (i.e. a king or queen)
I know of a few that are of religious origin, like the Achilles's heel after Greek mythology, and the bumpy bit of your throat is called Adam's apple is biblical.
Bob's Your Uncle is a pretty fascinating phrase, because it doesn't have a clear origin.
The earliest references to it are as a song title, mentioned in advertisements and later in a newspaper review, but neither the actual music or lyrics are known today.
Very soon after this first appearance, it starts appearing in print as a phrase people are using.