I always assumed that they were called “Tom Swiftys” because they were very brief. Swift, as it were, although the frequent use of adverbs in the Tom Swift books (if that was the case – I never read them) would’ve helped it along.
Just so’s you know, by the way, the “Tom Swift” books most of us are familiar with isn’t the original series. Those high-tech books with names like “Tom Swift and his Gravitational AeroBathySphere” by “Victor Appleton III” date from the 1950s, and are in imitation of the original circa 1910 series with much less ambitious titles like “Tom Swift and his Electric Bicycle” by “Victor Appleton”, the putative grandfather of V.A. III. All the Victor Appletons are, of course, house names disguiising a variety of authors. The 1950s vintage Tom Swift is supposed to be Tom Swift, Jr.
And th tradition of Boy Inventors long predates both Tom Swifts – “Boy’s Papers” of the 1870s featured the adventures of Frank Reede and his Mechanical Man of the Prairies and the like.
“I need new shoes,” said Tom, archly.
“I made it out of a copper alloy,” said Tom, brazenly.