When buying a new home, dealing with a recommended lender is often very important. This is because there are a lot of intricacies involved in new homes that do not exist when buying resale. If you "shop" around to find your own lender, you may end up with someone who quotes a great rate and is great with refinances or resales, but has no experience with new homes. This can lead to problems or delays.
Over the last ten years, real estate companies and builders have built up their own mortgage brokerages. "Bundled services" like this make sense because it adds another profit center to the company. This is useful because it helps real estate companies to offset higher commission splits with their agents.