People think weight = quality.

Sometimes it can indicate something is better made, like something made with lots of plastic vs more metal. In a PSU you need lots of metal for the windings, cores, and power stabilization components. It should have some heft to it.

Unscrupulous manufacturers will sometimes throw chunks of metal into an item (like Beats headphones) that do nothing except make a thing weigh more to prey on the sense that weight means better quality.

Feels more premium

In certain devices (batteries and power supplies) there's a minimum weight that can realistically store or convert a specified amount of energy or power.

So if you buy a 1000w PSU and it's too light, you're going to know it is fake. So they add the weight to make it feel right for the power rating. In this case this is a double-whammy of a failure waiting to happen. A PSU with a lower than advertised rating, coupled with a lack of safety circuitry means it's more likely to fail due to the overload applied, and when it fails it's more likely to go out in a big way.