Some of the keyservers work quite well. However many of them go offline for longer periods and from time to time you will see one of the long-time keyserver to be switched-off. It is something a geek is able to manage but it needs too much knowledge. With the ~2 million keys and the low traffic we have today you may have not noticed problems. However, we aim for hundred of millions of keys.
I contrast, DNS is a replicated and well matured database with enough people knowing how to maintain the system. It just works. There is even a standard for the key storage; something we do not have for keyservers (the de-facto standard for keyservers is the latest SKS version).
Another really important feature of DNS is that it allows for key revocation or rollover. Keyservers are not able to do this.