The max. limit of input current and voltage stated in the datasheet has to do with the reliability and safe operating area for the MOS input stage.

If the user forces more than the state current (meaning, that current has no other way to go than the gate of the input stage), the manufacturer cannot guarantee your amplifier will keep working properly. You'll probably stress and compromise the reliability of the gate oxide and, most likely, the input stage transistor will stop working as such.

Also, all MOS transistors need an input current to properly function, however small they are. For a 28nm CMOS process, 5nA of input bias current is not unheard of.

The max. limit of input current and voltage stated in the datasheet has to do with the reliability and safe operating area for the MOS input stage.

If the user forces more than the state current (meaning, that current has no other way to go than the gate/ESD input circuitry of the input stage), the manufacturer cannot guarantee your amplifier will keep working properly. The diodes will start to conduct, will clamp to a voltage and no useful signal will reach your input gate. You might even break/stress the wiring or ESD circuitry as a consequence.

Also, all MOS transistors need an input current to properly function, however small they are. For a 28nm CMOS process, 5nA of input bias current is not unheard of.