The reason is that if they have a claim and have to help the customer try to settle it with UPS, it can take dozens of phone calls to UPS, hours of time, and months to settle the claim. All this, only if they don't get mad at your persistence and hang up on you. They have no problem lying to you repeatedly just so they don't have to pay. We have taken to logging all calls to UPS because they can never keep their story straight. One time we had a rep tell us that we only entered the package in our computer log and that it was never picked up by the driver.
Guess what, we are one of the few people left on the planet that still use the paper log.
That is why Mailboxes is expensive.