The reason for this process, while frustrating for you as a buyer, is that even with all the regulations and changes that have been made in the lending industry there are still loan officers that will issue a pre-approval letter based on incomplete data or verbal information. Basically, they aren't worth the paper they are printed on and often, if accepted, those offers do not close.
The listing agents job is to protect thier sellers and they want to ensure that the offer the seller accepts will close. So... while it can be inconvenient, it is for the right reasons. Also, it's important for you to know that it's your choice in the end. You can work with whomever you prefer.
Happy home hunting and I hope this helped.
If a homeowner asks you to be pre-qualified with their favorite mortgage broker, I would ask why and flat out ask them if they are related to this person. And ask your broker about the legality of this! I have never heard of this in 24 years in the biz.
It's common practice for home builders to request buyers be pre-qualified by their mortgage provider, whether it's the first, second, or third pre-qualification done by that buyer.