The main reason why you will alway have a low voltage battery is safety:
It is used to actuate the contactors for the HV Battery.
You would not want un-switchable HV components.
For example in the event of a crash the HV battery gets completely disconnected but airbags and indicators will still work.
The 12v battery will get charged off the main hv battery if it gets low btw.
to piggyback off this, 12V components are also everywhere else, so you can use off the shelf components like lights, wiper motors, etc without having a limited production run for high voltage components, which keeps costs down. Doing it this way made it possible to add hybrid systems to traditional vehicles like a camery or an escape. This also enables you to jumpstart a hybrid with another 12V system.