What's the need for voltage detector ICs for batteries/supercapacitors?
Reliability: When you need to detect a supply voltage drop and need to turn of e.g. all the machinery, then you don't want to rely on a microcontroller which might already be misbehaving because of undervoltage.

Also, especially the KA75XX are from a time where microcontrollers even lacked basic things like brownout detection and couldn't reset themselves. That's why they needed external circuitry to do that.