Speaking as a martial artist with ki-adjacent magic powers, it's because it takes around 90% of character downtime to maintain the skills necessary to deliver ki in the ways D&D monks deliver it. If everyone could do it, everyone would do it. But most folks aren't into the hours of bodywork and meditation it takes to make it work.
...I have questions.
Haha. Sealing the Breath is probably the earliest "magic" skill you learn. Though the most important skills lie in shutting down similar weak points on your body....I have questions.
Because wizards are all about complex things with many moving parts! How many moving parts does your typical sword have?
The same thing they do every night, Pinkie: Try to take over the world?
Have you heard the story about the daughter who asked her mother why she cut an inch off the pot roast?
They're by and for serious Tolkien fans.
I've read that it was because Vancian seemed the easiest way to put a limiter on what spellcasters can do, and my experience playing a variety of games (and armchair-designing alternative systems) makes that seem reasonable to me. But also, Jack Vance's stories were just bitchin'. If only D&D had adopted his spell-naming style in addition to the fire-and-forget part!
Class niche protection.
But no, it's a good question! My personal perspective is that ki seems to be such a limited resource, split over/shared by so many class abilities, compared to those of other classes, which are individually usable X times per day, that those other systems are just regarded as better.People like buckets. Even more, people like putting things into buckets. Also, people do not like to have their buckets be stole.
