Sam2, Chatzi Nezek is also a Gezeiras Hakosuv. Will you say there is no reason? But anyhow, wherever an Eid Echad is Kosher, so is a woman. In other words, we have no problem believing her. She is just not a Kosher Eid.
The Ben Ish Chai gives a few reasons why she is Pasul. Keep in mind, though, that Eidus in itself is a Gezeiras Hakosuv. There are times that even when Beis Din saw something themselves, we need two other people to be Eidim.