Because they aren't giving you your money back, they spent that money.

What they are doing is giving you new money they just printed out of thin air.

New money derives it's value from the depreciation of existing money. So they are paying you with money that is NEWLY STOLEN from your bank accounts, investments, and property holdings. Inflation is a hidden tax. In so far as 'taxation is theft', inflation is the thieviest of them all.