A Fighter, a Rogue, a Monk, and a Barbarian are all Martial characters.
They move and they hit things. That's their baseline mechanic. But HOW they move and hit things tells a story that gets you invested into a specific character fantasy. Rogues do a lot more moving and ranged attacking because the damage dropoff for increased survivability is made up for with Sneak Attack damage. Barbarians wear light or no armor and wade into melee with big hit dice while raging to take half damage to offset the risks while landing a few BIG HITS. Fighters and monks both attack many times in a round, though the monk does it while also enjoying high movement and suffering a lower amount of HP and Armor options.
A Psion, a Sorcerer, and a Wizard all cast spells. And the vast majority of the spells they cast are exactly the same spells. (Especially since all things being equal some spells are more effective than others)
So how do they do it differently?
Wizards get the ability to recover spell slots at level 1! They also learn to Memorize a spell at level 5 to swap out one prepared spell every short rest. Other than that they just cast the spells.
Sorcerers can 'unleash their magic' for 1 minute to increase their spell save DC by 1 and gain advantage on spell attack rolls twice per long rest. Then at level 2 they can recover spell slots for Sorcery Points. And then they get Metamagic options. At level 5 they get to recover half their sorcerer level in sorcery points.
Psions do it more Subtly. And they get a wholly separate ability to use Energy Dice to do variable-range telepathy for a minute or fling an enemy a variable distance. This is not spellcasting, per-se, but it's something. And twice per long rest they can enter a 1 minute long state where they ignore psychic resistance (or gain it) and reroll dice by expending energy dice.
Sorcerers and Psions both get "Rage" for spellcasting, but much weaker and less built into the class than a Barbarian's Rage is. And then the ability to spend their sorcery points or energy dice on something that changes what their spells do... in practically the same ways.
All three have d6 hit dice, light or no armor, minimal weapon proficiencies, the same movement, the same positioning requirements... They're all so incredibly similar that there's very little that distinguishes them from one another in actual gameplay.
A Sorcerer, a Wizard, and a Psion in the same situation are probably going to move the same way and cast the same spell, or very similar spells, and end their turn. MAYBE they'll use an archetype ability or something as a bonus action for zest... but it's pretty much going to be the same.
There's not a lot of granularity between them because of how much focus is put onto the spells, themselves. Like if Fighter, Barbarian, Rogue, and Monk were all defined not by class ability, but by which weapons they carried, and they all pretty much carried the same weapon options.
