Per Gregory Clark in A Farewell to Alms, the ancients had noticeably less future time-orientation than modern people. Furthermore, there were relatively few ways to make profitable investments - it's not as though a farmer could take loans out to buy a tractor.

Did we read the same book? Clark's whole point was that there were many secure (eg. his argument that property rights were more secure in early Britain than during the Industrial Revolution) high-paying investments; this surprised me so much that I recorded one snippet from chapter 9 in my Evernotes:

All societies before 1400 for which we have sufficient evidence to calculate interest rates show high rates by modern standards [with ~0% inflation].5 In ancient Greece loans secured by real estate generated returns of close to 10 percent on average all the way from the fifth century to the second century BC. The temple of Delos, which received a steady inflow of funds in offerings, invested them at a standard 10 percent mortgage rate throughout this period.6 Land in Roman Egypt in the first three centuries AD produced a typical return of 9–10 percent. Loans secured by land typically earned an even higher return of 12 percent.7

... Medieval India had similarly high interest rates. Hindu law books of the first to ninth centuries AD allow interest of 15 percent of the face amount of loans secured by pledges of property, and 24–30 percent of loans with only personal security. Inscriptions recording perpetual temple endowments from the tenth century AD in southern India show a typical income yield of 15 percent of the investment.8 The return on these temple investments in southern India was still at least 10 percent in 1535–47, much higher than European interest rates by this time. At Tirupati Temple at the time of the Vijayanagar Empire the temple invested in irrigation improvements at a 10 percent return to the object of the donor. But since the temple only collected 63 percent on average of the rent of the irrigated land, the social return from these investments was as high as 16 percent.9

While the rates quoted above are high, those quoted for earlier agrarian economies are even higher. In Sumer, the precursor to ancient Babylonia, between 3000 and 1900 BC rates of interest on loans of silver (as opposed to grain) were 20–25 percent. In Babylonia between 1900 and 732 BC the normal rates of return on loans of silver were 10–25 percent.10 In the sixth century BC the average rate on a sample of loans in Babylonia was 16–20 percent, even though these loans were typically secured by houses and other property. In the Ottoman Empire in the sixteenth century debt cases brought to court revealed interest rates of 10–20 percent.11

EDIT: Adam Smith in The Wealth of Nations:

In Bengal, money is frequently lent to the farmers at forty, fifty, and sixty per cent. Twelve per cent, is said to be the common interest of money in China, and the ordinary profits of stock must be sufficient to afford this large interest.