Wholesale lettuce price yesterday was $10, so retail $12-13, not much of a margin.
But Banks wholesale for money is 1.35%, but sell it on at 4.8%
All comes down to scale.
So at the risk of being attacked by bank bashers...
If you look at the total profit (which would include profit from other non-lending sources like FX trading) of (for example) CommBank ($8.65b in 2021) against the amount of money invested in home loans (approx $510b) to achieve that profit, you'll find the profit margin for a bank on loans is somewhat less than 1.5%.
Compare that to your lettuce example where the margin is 20-30% and (being generous) the profit margin is probably still at least 10%.
Simon, That bank actually owns very little, sold most of their own buildings, get a lot of " their" money from fools like most of us and then broker a big percentage to hopefuls to pay back for A long time, so I dont get the return on investment calculation by you. So maybe I am dense ?