One reason is that databases are about data, and objects are about structures and algorithms. Once you take the data and embed it in classes, you characterize the relationships and operations in a static structure. Databases, on the other hand, are about unstructuring the data into a bunch of instances of atomic tables that can be reassembled into different structures (usually with classes) without disturbing the integrity of the atoms.
Databases are somewhat analogous to hexahexaflexagons.
