1 John  3:9

“Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.”

There is a “for” and a “because” in that verse. They give the reason why they don’t commit sin. After the first statement about committing sin, you should see that the second one is actually something like this: and he cannot COMMIT SIN …. BECAUSE…(His mouth will hang if he tries to lie?”)
Because he is born of God

He first says FOR HIS SEED REMAINS IN HIM.
His seed being in you is the guarantee of never sinning.

Sin, there, is to not have his seed in you. Sin there is to not be born of him. The one born of him cannot sin because a birth cannot be reversed

It is a matter of IMPOSSIBILITY.