Abandoned storage lockers

Why are storage lockers abandoned if there worth so much? Why is it legal to sell someone elses' stuff?

When you sign the contract and fail to follow through with your end, its no longer "someone elses' stuff".
I abandoned mine because it was 1000 miles away and the things that were in it weren't of enough value for me to haul from michigan to georgia.

People also die and family members are unaware of the locker.

I lost mine due to finances and it being a 3 hour drive away. Had lots of Scuba Equipment in it.

Some people go to jail. If they already have their property in a storage unit, it is
difficult to make a payment from behind bars. If they went to jail and had a
relative or friend move their property from their home into storage and the relative
or friend gets behind on the payment, their property is unknowingly sold.

Some people die and their family members are not aware that the deceased had
property in a storage unit. Others lose their jobs or become ill and fall behind on
their payments. It only takes a few months before the costs really add up and some
people just can’t afford to get current.

In other less tragic scenarios, a person may have to relocate on short notice. They
pack in a hurry and everything but the essentials go into storage. Over time these
items become less important and the cost to transport them to another state is
overwhelming, so they just relinquish the property to the facility or quit making
payments. Also, when people relocate, their addresses & phone numbers change.
If there is a problem with their credit card and the facility is unable to reach the
tenant, notices are placed in the newspaper and then their property is sold.

Depending on state law (varies by state) once a storage unit has followed procedure they may sale the contents of the locker to attempt to re-coup the loss.  The same applies to vehicles and such that are left at body and repair shops.  It's a smaller scale of your house forclosures.

tclane posted most of the reasons why.  I know in my area; being military, a number of the lockers go up due to troops being sent on deployment or such and not paying the bills.  IMO that sucks but it's a reality.