If a GK is binding part of their soul to a weapon, its gonna be hostile to someone corrupted- like a demon weapon. Throne mechaicum is hostile to the weak/ evil for Imperial Knights, souls of past pilots too. Same thing. Someone warp touched, god marked, chosen etc wold be safer with binding demons to wargear, than risking fighting a pure soul in a GK weapon to subvert it. GK weapon is basically an Imperial demon weapon that is hostile to chaos instead.
As opposed to Chaos daemon weapons, that are famously friendly to their wielders.
I'd assume it's far easier to corrupt something like this, and far less risky, than to bind an entirely new daemon into a different weapon.
A demon is wicked as is the one who would use one. A demon would only betray you at worst half the time because you have plenty in common. A pure soul would be constantly trying to destroy you, very likely do its best to resist its leaving/ destruction to kill you. You have more in common cause with a demon to make a deal/ understanding or even be the master as you are god marked, chosen, blessed etc. Its safer for evil to truck with evil. Word Bearers don't even use geller fields anymore when they warp travel such is their favor and power with the warp/ demons.