It's not just quality control on the part of the transformer manufacturer. It was a cheap design. They could've completely isolated the two transformer windings

It's not just quality control on the part of the transformer manufacturer. It was a cheap design. They could've completely isolated the two transformer windings, either with heavy tape, or wound them on two separate bobbins. That way, if the enamel on the wire wears off, the two sides are still completely isolated. I still agree with your conclusion, though.