If a player can not justify her actions in character, then there's a problem.
The thing is that
this is what descriptive mechanics are for, you dense
fuck. For example, first line of Enervation out of the SRD:
You point your finger and utter the incantation, releasing a black ray of crackling negative energy that suppresses the life force of any living creature it strikes. You must make a ranged
touch attack to hit. If the attack succeeds, the subject gains
1d4 negative levels.
The bolded section is a portion of the "how" for the mechanical effect. Almost nothing in D&D has
no flavor text, it's overwhelmingly at least
slightly descriptive. The justification for doing the effects of Enervate, in universe, is quite literally "I'm a Wizard casting a spell", which involves incantations and physical motions. The details of how that works are all over the place, because spell slots as they are in-game
really don't work in narratives and the underlying metaphysics isn't really in the ballpark of the great majority of writers, but on the level you described, it's sufficient.
The point I'm trying to make is that
there exists a how within the described game-setting combinations. What you are suggesting
is answered in-game, on a meaningful level, and as such is not
necessary to roleplay. Because licensed products exist, because setting-specific source books exist, because game-book fluff as a whole exists. If you want to roleplay in Star Wars, there's multiple licensed TTRPG systems for it that handle the great bulk of how you resolve the challenges, with all the fluff work being "you're playing a game in Star Wars", and as such what you are mentioning is
completely beside the point of the damn product. As it is for White Wolf products, which also merge the mechanics and fluff to a considerable degree.
The entire point of TTRPGs, as a
full industry, GURPS included, is offloading these sorts of questions onto a pre-defined rulebook to some extent. Inverting the questions, going from "how are you casting Fireball" to "what does casting Fireball do", pulls out the underlying
thing you seem unable to understand. That TTRPGs are a labor-saving device for roleplaying, giving
rules to structure all sorts of things. Which very much can extend to what you insist is a necessary skill for roleplayers, that of the "how" for mechanical effect. White Wolf products, licensed products of all stripes, D&D to varying degrees based on which mechanics you're using and what edition you're playing, a great deal of the TTRPG market is descriptive to the point where what you're calling a vital skill is entirely beside the point of the product.
The "How" isn't a vital
question, because you have a book of rules that is
capable of offering the answer right alongside the rules themselves. Being able to come up with the in-universe "how" is
not a vital skill,
because most systems pre-answer it, for their particular setting assumptions, to a sufficient degree as to be able to focus on what to do with those abilities, which is what is the actual role playing. You, however, have a visceral hatred of a major aspect of the system you're playing with (the default spellcasting), and keep using it for settings it doesn't have mechanics for (science fiction, high-Wuxia, various other such things that D&D doesn't describe
at all). So
the way you play means you're left with a great many square pegs for round holes, and are left with needing to do a great deal of fluff work because you keep going into things that don't have a presence in the system you're working with.
Even if it's just "martial mastery can be great, too", D&D is
not a system built for that, and rather than even do so much as whitelist homebrew Disciplines (you mentioned not taking home brew ToB stuff when you were listing off alternate magic systems), you practically throw out a third of the rule books to have players use any mechanic for any character concept, making for a greater amount of storytelling and DM legwork in the process by making the
worldbuilding need to cover the basic mechanics that the product is
specifically intended to be about covering.
And some people
do not work with needing to come up with the fluff. Roleplay and worldbuilding, which is what writing up fluff for mechanics is, are separate skill sets, and your position is that you're bad at the former if you don't like the latter.
