Assuming this is not a rhetorical question, the answer is: Buying stock is investing cash into a business. The investment has more risk of going down in value

Assuming this is not a rhetorical question, the answer is: Buying stock is investing cash into a business. The investment has more risk of going down in value compared to cash in the bank. Government created a tax advantage to partially offset the risk, and thus encourage investment. This tends to move capital to places it can be used for creating new things, instead of sitting in a vault.