The reason that UPS SurePost takes so long is it's a cheaper alternative to regular UPS shipping.
UPS SurePost packages are put on UPS trucks going in different directions than your normal UPS trucks would be going.
They fill the trucks up as much as they can to save money and so you get a discount on the shipping because UPS does not do the final delivery of the package.
When you have a package going through UPS SurePost it goes through UPS sorting facilities and travels on the UPS trucks which are semi trucks that go down the interstate and go from sorting facility to sorting facility.
Then when the UPS SurePost package gets to the destination facility the package then gets loaded onto another UPS truck and then is delivered to the USPS sorting facility.
Then your local USPS sorting facility delivers the package to your local post office.
So that's why UPS SurePost takes longer as it gets handed off to your local USPS Sorting facility which then gets delivered to your house or post office.
UPS SurePost takes an additional 3 to 4 days.
While regular UPS takes around 3 days.