This is like discussing why pigs get fat... answer is simple, it is not the pig's fault but the fault of whoever feeds the pig!
In the environments I control no developer does database design work. Period.
You want healthy databases? start at design time.
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Pablo (Paul) Berzukov
Author of Understanding Database Administration available at Amazon and other bookstores.
Disclaimer: Advice is provided to the best of my knowledge but no implicit or explicit warranties are provided. Since the advisor explicitly encourages testing any and all suggestions on a test non-production environment advisor should not held liable or responsible for any actions taken based on the given advice.