Why the reward of capital is interest?

The reward for capital is interest because it compensates lenders for the opportunity cost of forgoing their money's alternative uses, such as investing in other ventures or spending it. Interest reflects the risk associated with lending, as borrowers may default, and it also accounts for inflation, ensuring that the purchasing power of money is preserved over time. Additionally, interest serves as an incentive for saving, encouraging individuals and institutions to defer consumption in favor of future benefits.