Why do you think some sites like H&M, Zara, and 6pm don't let people post reviews? Is it because it drives up the cost of maintaining the site? Or some other reason? I also just noticed that Barneys doesn't have item reviews, so cost doesn't seem like the only reason.
Sometimes I'll search the web to see if people have reviewed the items elsewhere (with 6pm sometimes it works), or if they took a pic wearing the item, but for the most part it doesn't work.
I just ordered a pair of paints from Zara and had to return them, and I bet if there had been reviews I would have understood the fit better (the lanky model was definitely not an accurate representation of how the pants fit on most people).