Prada Designer

What is it about Prada/Chanel/etc. that makes them designer?
What really is it about D&G, Prada, Gucci, Chanel, etc. that makes them considered so high in quality?
Don't get me wrong, I adore their clothes. They last a long time and the material feels amazing.
The thing is, I can pratically find a replica of their clothes in simple department stores...once I actually found a similar dress of Barney's in Walmart. Thousands of dollars cheaper and it lasted forever. In fact, I still have it now. And it felt the exact same to myself as the "real" one.
So honestly, I don't get it. Call me cheap or whatnot, but I think that department stores have almost the exact same thing as those designer brands. So what exactly IS it about Prada, Chanel, and designer clothes that makes them...high quality and designer?
Yes, I understand about the designer. But why are they so great? I think those clothes should only be for a splurge, as you can find exact similar ones.
I guess it's all just about the popularity. Take a the name "Energie" and it;'s designer and compare it to like "Chanel" and it's designer. Which do you think people will recoginize? ....Chanel
Or, it could be things like the names being mentions in shows/songs/movies/ect. Take Lady Gaga's song "Fashion" for example.
Designer Shoes Shoes Shoes (Mostly Prada, Dolce & Gabbana and Gucci, but some others too!)
