A family member just went through this. The Visible rep claimed that it was because of the high volume of orders for the iPhone 13. However, I have a feeling it's related to their recent breach in which hackers were logging into accounts, changing passwords and then ordering phones. The long window of time before it ships would (theoretically) help it to stop these orders from processing if they were fraudulent.
It probably would have been more helpful to provide more details, such as what phone model was purchased.
With that said, in the last week, I've purchased an iPhone 13 mini and an iPhone 13 Pro and each shipped out same day of order and delivered next business day, as expected.