Speaking from the perspective of in campaign lore:
Sorcerers gain their power through the weave, while Psions do not. The source of power of a sorcerer is external to their body, while the psionic characters are generators of their own power. The sorcerer has a connection to a supernatural force that allows them to access the weave. However, they depend upon that weave access for their magic, and anti-magic, detection of magic, etc... all work with/for them because they are tied to the weave.
This is why the sorcerer is mechanically
similar to a wizard. They use their heritages to tap into the weave and gather power that they, like a bard, can shape with their force of personality (charisma) - but it is also why they are mechanically different from a psionic character - because those characters are not restricted, or benefited, by access to the weave.
- A cleric or paladin asks for magic, and it is given to them through the weave.
- A druid or ranger is awash in the weave, feeling the flow of the magic from the positive energy plane to the negative energy plane (and back), and just guides thet flow into their spells, working with the currents.
- A wizard reaches into the weave and pulls, manipulates and twists that magic to create their spells, using precision and focus to craft magic.
- A sorcerer opens up to the weave and draws magic in using the elements of their heritage (or whatever otherworldy influence permeates them), and through force of personality pushes it into shape.
- A bard doesn't have some supernatural origin element to help them push the magic into shape - they're all ego, baby, and they get it done by being who they are.
- A warlock is delivered the raw magic via the web, but must use their force of personality to shape it.
- A monk, psion, or psychic warrior don't look to the weave for their power. They look inside. They've trained their souls and spirits to create power without the aid of the weave, thus allowing them to do things that weave based casters can't - and also thus being unable to do things that weave casters can, as some key magical effects depend upon how magic flows from the weave.