I'd guess it's familiarity and encouraging sales. If I were to guess, it's wargaming like everything else. Wargames were popular as you control an entire army,

I'd guess it's familiarity and encouraging sales. If I were to guess, it's wargaming like everything else. Wargames were popular as you control an entire army, but D&D originally meant to answer the question of what things would look like if you controlled just one person in that army in the middle ages when the only major fantasy was stuff like LOTR or Narnia along with an inexperienced guy with no examples of good game design to look at. He borrows 6-sided dice for the game, but the guy who takes control of everything moves to polyhedral dice because nobody owns them and almost nobody makes them, so gamers will be forced to buy dice which have a huge profit margin. The fledgling system needs magic, but the big magic from LOTR or similar is simply overpowered and is a soft magic system (few rules). You have to reign this in so wizards aren't overpowered. One of the most naive ways is cooldowns just like a wargame might limit powerful guns by how often they can load and fire. Tracking a bunch of different cooldowns in-game is very bad in actual play, so you give them all one cooldown (one day), but now some effects are not worth carrying because they are too weak, so you allow those to be used N times per day which is much easier to track (non-temporal counting is easier). Just like that, you have Venetian magic. Why did it stick? I'd guess part is tradition, but the other part is book sales. Because balancing these spells is rather hard (lots of people have played that campaign where the GM allowed an "innocent" custom spell that then proceeded to break the game), most people find it easier to buy multiple spellbooks of already balanced spells. Even if just 10-20% of all gamers buy those books, that's a big increase in sales volume. In contrast, even the most elaborate effects systems out there (Hero and GURPS) fit their entire effects system into 50-100 pages and their only upsell is to GMs too lazy to create their own spells (or those of us who claim we aren't being lazy and just want to support them).