It comes from a bit of a tradition
Tom and Jack
The Dictionary of Phrase and Fable by E. Cobham Brewer says:
"Between “Tom” and “Jack” there is a vast difference. “Jack” is the sharp, shrewd, active fellow, but Tom the honest dullard. Counterfeits are “Jack,” but Toms are simply bulky examples of the ordinary sort, as Tomtoes. No one would think of calling the thick-headed, ponderous male cat a Jack, nor the pert, dexterous, thieving daw a “Tom.” The former is instinctively called a Tom-cat, and the latter a Jack-daw. The subject of “Jack” has been already set forth. (See Jack. ) Let us now see how Tom is used:-
Tom o' Bedlam (q.v.). A mendicant who levies charity on the plea of insanity.
Tom-cat. The male cat.
Tom Drum's entertainment. A very clumsy sort of horse-play.
Tom Farthing. A born fool.
Tom Fool. A clumsy, witless fool, fond of stupid practical jokes, but very different from a “Jack Pudding,” who is a wit and bit of a conjurer.
Tom Long. A lazy, dilatory sluggard.
Tom Lony. A simpleton.
Tom Noddy. A puffing, fuming, stupid creature, no more like a “Jack-a-dandy” than Bill Sikes to Sam Weller.
Tom Noodle. A mere nincompoop.
Tom the Piper's son. A poor stupid thief who got well basted, and blubbered like a booby.
Tom Thumb.A man cut short or stinted of his fair proportions. (For the Tom Thumb of nursery delight, see next page.
Tom Tidler. An occupant who finds it no easy matter to keep his own against sharper rivals. (See Tom Tidler's Ground.)
Tom Tiller. A hen-pecked husband.
Tom Tinker. The brawny, heavy blacksmith, with none of the wit and fun of a “Jack Tar,” who can tell a yarn to astonish all his native village.
Tom Tit. The “Tom Thumb” of birds.
Tom-Toe. The clumsy, bulky toe, “bulk without spirit vast.” Why the great toe? “For that being one o' the lowest, basest, poorest of this most wise rebellion, thou goest foremost.” (Shakespeare: Coriolanus, i. 1.)
Tom Tug. A waterman, who bears the same relation to a Jack Tar as a carthorse to an Arab. (See Tom Tug.)
Great Tom of Lincoln. A bell weighing 5 tons 8 cwt.
Mighty Tom of Oxford. A bell weighing 7 tons 12 cwt.
Old Tom. A heavy, strong, intoxicating sort of gin.
Long Tom. A huge water-jug.
Tom Fool's Colours Red and yellow, or scarlet and yellow, the colours of the ancient motley.
Tom Foolery The coarse, witless jokes of a Tom Fool"