Actually the IIe has a dedicated keyboard controller as well, it just happens to be on the main board instead of stuck to the back of the keyboard itself.
(Same part is used in the IIc and the Apple III. The IIgs uses a custom 6502-based MPU to both emulate this part and to drive other aspects of the ADB bus. One of my two IIgs has a bad one of those unfortunately; it's pretty much unobtainium short of desoldering one from a IIgs that's broken in some other way.)