Because we aren't trading actual goods, but notes that are "guaranteed" to be worth something. That's fiat money. You aren't trading actual gold coins, or even coins or notes guaranteed to represent a specific amount of gold or silver, but notes guaranteed to worth something as long as well still believe in them. It's a system of trust, and if enough people distrusted that the notes were worth anything, it would collapse (as has happened in other countries in the past).

So, even if an entity had $1 in reserve, but gave you notes representing $1,000,000, that money is valid as long as both the person you are trying to pay with it trusts that the issuing entity will honor the note, and it is worth the trade being offered (services, goods, etc). It might not be a good idea to trust that entity, but if the system persists long enough, and the entity makes enough money on fees and interest to actually cover the extravagant lending before people try to claim the notes, then in the end it was (or may be, depending on the parties involved) a net success. The issuing entity made money on the loan, the person taking out the loan was able to use it to provide needed liquidity (hopefully leveraged to make their situation better), and the person paid with the notes was able to redeem them at the issuing entity, or use them with other people successfully.

What we have currently is that the government issues, but does not want to be responsible for vetting individual loans, so only loans to large entities that then re-lend (and these lenders may take deposits as a bank or have other sources of cash to lend out). They also require these lending entities keep a portion of the amount they lend in reserve (the $1 from above), but at a certain percentage rate such that they expect in most instances the re-lending institution can weather most cases of increased demand to make good on the money for their loans (otherwise a lender could over-leverage themselves with far too much lent to deal with even normal economic events).