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.So why are Monks the only people using this ability? At the very least, you'd expect ki-using subclasses to exist for other classes!
Honestly, spending a couple of hours a day on spear-play or hacking drills is much, much more bang for the buck.