Hello... In addition to builders the answer is YES there are listing agents who are required (by the seller) to have buyers approved by thier (the sellers) preferred or approved lender. If the property is owned by a bank (foreclosure) or by a non-profit it is common. In addition, on a rare occasion, a listing agent may request this from potential buyers strictly for the benefit of thier sellers and not because they are required to.
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.