They survive on customers who are at work while the builders providers are open, or who want to browse products on display or don't feel comfortable asking in a builders providers. There's also the convenience factor of everything under one roof. But yeah, you pay dearly for it.
A lot of the builders providers were less than welcoming to DIYers during the boom, skipping customers to deal with the builders they knew. They had little time for anyone who wanted help. Some of these businesses have gone under now, and the ones that remain, that I have experience of at least, are a little more welcoming.