Its the same as for not folding bikes.

The lower weight folding bike have a frame that weight less due to the use of higher end material (high end steel, titanium but carbon, up to now, didn't prove to result in lower weight for folding bikes) and more sophisticated manufacturing process (butted tubes, hydroformed frame...).

And, like lightweight not folding bikes, they have lower weight, more high end and more expensive components.

But its true that a folding frame weight more than a not folding frame because the folding mechanism add some weight and the fact that the frame is folding requires some frame reinforcement that also add some weight.