Why ATX PSU is using separate ground wires?
ATX PSUs use a lot of ground wires because the system has a lot of separate loads, and every feed needs a return path. The chassis are usually made of ferrous sheet metal, and have higher resistance than copper. And yes, there could be unwanted ground offsets and cross-talks between nets with different consumers.  Instead of using a single finger-thick-size return ground, ATX supply uses many smaller ground wires, which distribute the return current more evenly, and make the cabling more flexible and manageable.