Money has an insurance effect. Simply having money lets you pay off random expenses. It also doesn't spoil like goods. If we had a grain money system, then the central bank would pass the cost of grain storage onto the holder of the dollar bill which would give the lender an incentive to lend it out even at 0% interest, which protects the lender from the storage costs. This is known as liquidity preference. The borrower has to compensate the lender for giving up the liquidity benefits of money.
Right now cash itself has no storage costs, so nobody will lend it at 0% interest.
In theory giving the rich money through lower taxes lowers the interest rate because they lend it out. However, as I said, there is no mechanism that actually lets it fall to 0% (extreme cases like deflation can in theory justify negative rates). If you were to charge a negative interest rate on a bank account to enable 0% loans, then people just pull their money out and hold cash instead.
The free market basically ends the moment markets are saturated and everything only makes enough money to cover its own costs with 0% profit. This is one of the areas where Marx was correct. Once profit is gone, the system collapses because someone forgot to enable the 0% interest feature of money.