Distance. We do not have a nearby UPS Store in Michigan. The nearest is 17 miles away in Ohio.
There is a drop box in my town (have to box and label), but sometimes it costs to return.
Most of the time Amazon provides me with 3 options for returns, ups, Khols department store or Whole Foods grocery stores.
Fortunately for me I have a khols and ups store less than 2 miles from me and they are literally next door to each other.