The reason those cables get hot is the same reason that you got them so (relatively) cheap: metal is a significant fraction of the cost of a cable. The manufacturer who produced the cable knew exactly what their customer was looking for, and provided a low cost cable with thin conductors.
I've had USB cables get hot before. The finest being a cheap Chinese USB cup warmer. Why they build such things escapes me.
The losses at low voltages are pretty high as well.
There's a reason that we have mains cables how they are and our current power distribution infrastructure.