Ill fitting bikes sold by sleazy shops and material are two separate issues. They are just being discussed in the same thread. The OP questioned some relative merits about different materials (steels, carbon, alu) in her post, in addition to the size/fit issue.
Why do bike shops sell the wrong sized bike?
Because they want to make a sale. It is never the customer's fault. When many people go to an LBS to buy a bike, they don't know what to ask and what to try. They may not be familiar with all of language about components or geometry. Trying out a bike in the parking lot or on a trainer is not enough. You need at least a 10-15 mile ride. Shops tend to do this with women, but it happens to men, too.
A friend of mine just bought a Specialized road bike, after riding a flat bar road bike. We sent her to a shop we knew would have what she needed and is not pushy. Still, if she hadn't been armed with the questions and information myself and DH spent hours going over with her, I am sure she would have been screwed. She got a crash course in components, shifters, gear ratios and it paid off. She has a beautiful bike she loves.
2015 Trek Silque SSL
Specialized Oura
2011 Guru Praemio
Specialized Oura
2017 Specialized Ariel Sport