I deconstruct clothing to get fabric of a higher quality than I could afford as yardage. Without a doubt, expensive brand name clothing is made of better fabric. Woolrich, L.L. Bean, for example are made of great stuff. Clothing bearing low-end labels are often--though not always--not worth bothering with. Why is this? There are more details and no skimpy seams in the good clothing. The cheaper garments usually have less detailed sewing but of good craftsmanship. The difference is in the quality of goods used.
When I worked in a sewing factory I was quite surprised when the boss educated me on the expenses of clothing manufacturing--the costliest component is the cloth (NOT the labor, as I thought).
This is a parallel to what other posters have pointed out. Retailers who wish to make a profit selling at low prices are more likely to carry attractive clothing or yard goods from less durable raw material.
It's delicious to go to a real quilt shop and feast on the lovely array so beautifully displayed and make a purchase--like a child with a nickel in an old-time candy shop--but I collect fabrics where ever I can. To me, it's a pleasant challenge to make something useful/attractive from these oddments.