It's the catch-22. If you make an entirely new mechanical framework, then it makes psionics different and interesting than spellcasting but is much less likely to mesh with the game. And many players will just find it off-putting and too difficult to incorporate so they'll ignore it... or they'll question why the exact same ability like detecting thoughts works one way as a spell and another way as a discipline.
The other method is to make psionics talents spells because everything in the game is already spell-based, even if they are fluffed as more natural abilities rather than spells. This makes it much easier for players to understand how they work, and how they balance themselves in the grand scheme of supernatural abilities, but make them much harder to fluff as something different.
It's the same issue we already have with things like the 4 Element Monk's magical abilities, or the Warlock's invocations. The game tries to make them seem different by giving them their own special names to denote in the story that they're not supposed to be just "casting X spell"... but they are using their magical power to invoke the Fist of Four Thunders or the Mask of Many Faces. When in truth they're just casting Thunderwave and Disguise Self just like the other members of the party. It's meant to be different, but really it plays out exactly the same. At least if you had a different mechanical framework it might feel different when you used the ability even if the effect matched. Just like how the Champion's expanded crit range feels different than the Ranger's Hunter's Mark, even though in both cases the player is just adding an extra die to their damage at various points in the fight.
This is really why psionics has never really been a thing I've felt was missing from the game, because I've never seen psionics different than just the various psychic spells that any other magical class already does. Being a psionic has always felt more like a descriptor of a person who focuses on a certain type of supernatural ability (psychic), just like somene saying they're a pyromancer (one who focuses on fire) or healer (one focused on healing magic). You don't need a new mechanical class or framework to be a pyromancer, so having one just for psionics has always felt unnecessary to me personally.