And even when we talk about why the loans defaulted, the issue isn't that the loans were subprime instead of prime and that that was due to a higher risk of default. It's that the loan was given based on falsified data to cover up reasons that should have prevented the loan from being given (almost always due to a debt:income ratio that was much too high), the borrower was advised by fiduciaries to take more than they should have given their income, and the preferred loan type was one where the the necessary payment ballooned massively after a set time period, which meant that when the payment when up the borrower would be unable to pay the higher payment because of how far they were stretched by the base payment, because the borrower was advised that they would be able to refinance at a lower rate due to an expected-by-their-fiduciary rise in equity.
So the already-risky loans were made more risky due to fraud and deceit on the part of loan writers. Fair enough, I have no argument with that as a major contributor, if not root cause.
That way over-simplifies, and misses a key point: the loans don't "have the best interest rate" because they are assessed to be loans at higher risk of default. The way you say, it sounds like the house buyers just didn't shop around enough for mortgages.