Why is upholstery so expensive ?

Reupholstery and upholstery cost is all relevant to the quality of the materials used and the type of upholstery that is involved in any particular piece of furniture. For example an antique chair using tacks and other traditional techniques and materials of upholstery will be more labor intensive than its modern day foam and staple equivalent.

You will also find that the price of one new chair or sofa vs the cost of some others will vary hugely. This is also partly to do with the quality of the materials used. You might have one frame made from chipboard the other from solid beach with maybe walnut, glass, or engineered steel legs and latex foam instead of some other cheaper alternatives. 

Cushion fillings will also affect the cost of a piece of furniture for example 1kg of duck feathers will cost around £4 whereas if you would like down feathers this would cost £104 per kg

As you can see there are vast differences in types of labor and material costs.