Borrowers tend to be less wealthy, so when they receive an incremental dollar, they tend to spend it. This creates demand, and stimulates investment to meet that demand. Lenders are generally the wealthy, or governments of countries with excess savings such as Germany and China. Such lenders tend to save dollars they receive — adding to the global savings glut, rather than stimulating demand or investment.
An economics in which lowering the price does not increase demand. Lowering the price of capital does not increase investment. Fascinating, isn’t it?