Game lore basis for verbal components: Blame Jack Vance
It was assumed that the D & D spell would be primarily verbal ~ E. Gary Gygax, 1976
The original reason for verbal components had nothing to do with "the Weave." D&D 5e is an interesting mix of all editions previous to it.
... the particular combination of sounds, with specific pitch and
resonance, sets the threads of magic in motion
In game, the caster would feel it as he spoke the words.
The citation above from PHB p. 203, of how words act as a trigger to releasing magical energy from the weave, is an adaptation to the current canon of the old tried and true magic system that D&D's had for 40+ years. It is close enough to the original system that the concepts do not contradict each other.
Formalizing verbal spell components in the rules was part of adding all three spell components - V, S, M - to spell casting in AD&D 1e as part of a move toward more detail (simulationism) in the game's design.1 It's still with us. (Without that basic feature of magic, Silence as a spell is a lot less useful). The requirement for verbal components go back to the original D&D magic system, and how magic worked in Jack Vance's Dying Earth books. Discussions on Vancian magic can be found here, here, and here. From the last link, you can see how "the weave" looks a lot like the "universal energy" of Vance's world:
In this world, magic taps into some kind of universal energy. Cast
spells require the caster to memorize cryptic and powerful words that
almost seem to have a life and energy of their own. Just memorizing
the words is an act of magic in and of itself. Even one of the more
powerful wizards in the story can only remember a handful of such
spells at a time without losing them.
In OD&D, if all you had was books you knew there was magic but you didn't know "how it worked" in any kind of detail. The D&D magic system wasn't explained in the books, but was explained in a Strategic Review article (April 1976, pages 3 and 4) as a response to the questions sent in by many players and DM's. (At least 5e offers an explanation).
(E.Gary Gygax, Strategic Review, April 1976, p. 3-4, The Dungeons and Dragons Magic System (extract)
Because there are many legendary and authored systems of magic, many questions about the system of magic used in D&D are continually raised. Magic in CHAINMAIL was fairly brief ... limited to the concept of table top miniatures battles ... a somewhat different concept of magic had to be devised to employ with the D&D campaign in order to make it all work.
The four cardinal types of magic are
- those systems which require long conjuration with much paraphernalia
as an adjunct (as used by Shakespeare in MACBETH or as typically
written about by Robert E. Howard in his “Conan” yarns)
- the relatively short spoken spell (as in Finnish mythology or as
found in the superb fantasy of Jack Vance),
- ultra-powerful (if not always correct) magic (typical of deCamp & Pratt in their classic “Harold Shea” stories),
- the generally weak and relatively ineffectual magic (as found in
J.R.R. Tolkien’s work).
The basic assumption, then, was that D & D magic worked on a “Vancian” system and if used correctly would be a highly powerful and effective force. There are also four basic parts to magic:
- The verbal or uttered spell
- the somatic or physical movement required for the conjuration
- the psychic or mental attitude necessary to cast the spell
- the material adjuncts by which the spell, can be completed (to cite
an obvious example, water to raise a water elemental).
It was assumed that the D & D spell would be primarily verbal, although in some instances the spell would require some somatic component also (a fire ball being an outstanding example). The psychic per se would play little part in the basic magic system, but a corollary, mnemonics, would. The least part of magic would be the material aids required ...
The influences that shaped D&D originally were an eclectic mix of the literary and the cinematic. As Gygax noted in his article, verbal components have been associated with magic for a long time. Stage magicians used spoken "magical" words like "Abbracadabra!" or "Presto Change-O!" while doing tricks on stage, back in the day.
An example of the cinematic magic of this style is Merlin's charm of making in the movie Excalibur2. The idea that wizards, witches, and magicians spoke words of power to cast spells well predates the invention of D&D. You could also watch Sleeping Beauty or Cinderella, the original Disney animations, and see verbal components being used by the magic users as part of a spell. (Bibbity bobbity boo!)
1 There was a similar move made in that edition with weapon "to hit" modifiers by weapon type versus armor class -- a longsword got a minus to hit AC 2 (plate and shield) while a mace got a plus.
2 Excalibur was released after AD&D PHB came out, but I have no evidence that AD&D influenced its production.