Well, mortgage bubbles in particular tend to be accompanied by mortgage fraud.

One of the biggest factors in the global financial crisis was so-called accounting control fraud: executive compensation was (is) based on quarterly revenue, and executives could fraudulently increase quarterly income by issuing a ton of loans with negative expected returns. The bank might go under in five years, but why does the executive care? Then, on the flip side, brokers are paid on commission, so they have strong incentives to make sure that mortgages make it. I point this out to say that neither morals nor "rational self-interest" represented serious constraints on the growth of lending, because the problem with the argument from self-interest is that it's usually considered with respect to the wrong person's interests.

Anyway, during the later days of the real estate bubble of the '00s, the focus for particularly bad lending shifted to loans in which banks did not actually verify the applicants' stated income. (This is normally reserved for people who the bank has good reason to believe are actually good for it.)

So I suppose the point is, in principle this could represent a higher constraint, but in practice, "where there's a will, there's a way."