Unlike numbers on a bank account, which represent legal liability of the bank and are backed by the collateral of specified real estate or other debt instrument that a borrower offers as a way for a bank to secure the loan, the numbers on a bitcoin address are completely void of any legal liability since software that creates them is not a legal entity which can have legal rights and obligations.

So basically, people are paying huge amounts of money to get some empty numbers which are then recorded in a public ledger on the internet. If nobody wants to accept these numbers in exchange for dollars, goods or services, they are as useless as a random number that I write on a piece of paper.

On the other hand, since dollars in a bank account are bank's liability, if nobody wants to accept dollars in exchange for goods and services, they will be settled over time through the legal system by transferring real assets from those who have received loans from the bank, since loans are secured by the collateral of specified immovable or movable property and they represent bank's assets. This comes from the fact that obligation to repay the debt would not vanish if nobody wants to accept dollars. Since the borrower received real assets for its dollars, in the absence of dollars the borrower's debt must also be settled by real assets. Alternatively, since dollars that are in circulation were exchanged for goods and services by some borrower and since he is obligated to repay his debt in dollars, if nobody wants to accept dollars the borrower is the one who is forced to accept them in exchange for goods and services otherwise he is unable to settle his obligation to the bank. Besides being backed by borrower's collateral, dollars are also backed by banks' capital and the ability to pay taxes. As we can see, dollars on their own, have real measurable value, just like goods or services.

Bitcoins on the other hand, don't have value on their own, and if nobody wants to accept them in exchange for dollars, goods or services, no legal action can be taken by their 'owners' to get some kind of value out of them. As such, Bitcons are just empty numbers on the internet, which means that people are exchanging real values for literally nothing.