It's because they use a shipping partner. OSM picks up the packages in Florida, they get trucked to the main OSM facility in Illinois, then they go to the correct USPS Hub. This really delays the process.
For example, had my two packages (shipped Jan 13 and Jan 14 respectively) been shipped directly with USPS instead of shipping partner, they would have gone Davenport, FL-Jacksonville, FL - Atlanta, GA - My local post office and would have been here in three-four days. Instead, it's been 7 days and they're just now leaving OSM.
I actually called OSM and before I could even give her a tracking number she asked "Is it a Disney package?" That leads me to believe this is a pretty common issue for them. I don't understand why they just don't use USPS directly and cut out the middle man. It would save SO much time.