Here's a more in-depth answer. Lenders don't trust lending among one another, and you're asking on of them to underwrite a loan for you without determining the current market value of the underlying collateral simply because you have a credit score above 700. I'll use an analogy to help illustrate my point further. Let's say someone, who has a 750 credit score, asks to borrow $10K from you to purchase a burger from McDonald's. Does that loan make any sense? Would you lend him/her the $10K for that purchase? I hope your answers to both questions are "no", and they'd certainly be "no" if s/he approached a conventional lender which such loan requests.
Also keep in mind that drive-by appraisals were more common when banks had more control over who performed the appraisal. Now that they can't use their own people--for better or worse--they're not willing to expose themselves to more liability by not conducting their due diligence thoroughly. Keep in mind that investors who got burned, the government, and others are already engaged in several witch-hunts (deservedly so in some cases); so the banks had to become more conservative with their due diligence.... more