I think that there are two philosophical reasons.
First, people traditionally tend to separate persistence from real functionality. Once you strip away an object's "life" away from it and keep it primarily for persistence, it becomes a record, and then there is a tendency to treat it as a "lifeless" data object.
Following on that, when people think of a large collection of very similar things, they start thinking of them as tables rather than objects.
I think with O/R the distinction is starting to disappear. For example, I use hibernate to dump really complex class hierarchies into a MySQL database. However, I don't write performance-critical stuff for my project so I'm sure it's not done efficiently.
