Why is method invocation in Java polymorphic but field access isn't?

This general area is covered in Section 15.11.1 of the JLS. The designers of the Java programming language feel that the lack of dynamic lookup on field accesses provides greater run-time efficiency. In addition, if people want dynamic lookup on fields then they can easily use an accessor (aka "getter") instance method instead of direct field access. I.e., they leave the tradeoff up to the programmer.