IMHO the main reason is because 90% of residential real estate agents are terrible. They put listings up on the web and then sit on them either because they are not organized or too busy, or both. If I am very interested in a listing I will look up their license information from a given state and email them at their email on file there, and call their phone number from there - many of them are different than what is provided on the listing.
If I still can't get in touch I send an LOI with a purchase price. They are typically obligated to provide this to the client which can get the conversation going. As long as you have language in there this is a blind offer prior to reviewing the books, and make some reasonable assumptions based on what you have seen on Google, market, etc, it should be close unless there are surprises, which there always are.