The same reason why those who handle a lot of valuable assets have to post bonds or get insurance today. If money or valuables turn up missing, the bonds or insurance are there to make up the difference. If a mint official was careless or dishonest, they were civilly and criminally liable, but at least the bonds indemnified the depositors. The early mint had some gold and silver on hand to keep “the float” going. The vast majority of the gold and silver the mint handled came from the deposits that businesses and private individuals made to the mint to have those metals made into U.S. coins. If they were willing to wait a few months, the mint converted that metal into coins at no charge. If they needed the coins on short notice, there was a nominal charge, something like 5%. In same instances the mint received low purity deposits with cost a great deal to bring up to standard. This was why the first mint was a money loser for the early American government, which was why some people in Congress wanted to abolish it. The only way the mint really made any income or seignorage (a fancy word for the profit a mint makes on the coins it produces) was from its production of half cents and large cents.