Grocery stores don't require millions to billions of dollars of capital to execute each new transaction.

So it's not the margin itself but actually the spread between the margin and what investors could get by investing in alternatives. Real estate investment opportunities are often measured by their advantage (measured in fractions of a percentage, .2% advantage being considered solid) over 10 Year Treasuries or S&P 500 returns.

Real estate developers do often actually lose money, but the more salient boundary condition is whether they can get financing for a project, where they have to clear a bar well above the "just make >$0" bar.