Comment by marcinzm

The same reason Amazon runs its own shipping network instead of relying on UPS or Fedex. At a certain scale it is cheaper to get a custom option that specifically fits your needs rather than retro-fitting something else. The USPS bought 100,000 of the last truck and, for example, one of the requirements was long service life. Building a truck you can use for 30 years is probably much cheaper than buying multiple lower service life trucks over that period.