It is increasingly trivial to try millions or billions of hashes per second. Using scrypt forces the computer to use more resources (in this case, memory in particular) which means that scaling the guess rate is orders of magnitude slower and cannot be significantly increased by building faster computers (because it's limited by memory not process speed).

http://en.wikipedia.org/wiki/Scrypt

It comes down to this: Why use something which is designed for speed like hashes to do something that can be broken by highspeed guessing?

By brute-forcing it using GPUs. With a consumer-quality graphics card, anyone can try thousands of variations on every word in the dictionary in a few seconds. Common passwords that are not related to dictionary words (such as numbers and dates) take an ignorable amount of time longer.

While you may use better passwords than this, it's certain that some of your users do not.

The key is that you want an alg designed for passwords, not for checksums. The design goals of each are very different.