Hospital cash back Plan

I have a Hospital cash back insurance with FNB. I was hospitalised on the 15th September till 04 October for severe recurring depression and generalised Anxiety. I am still booked off till. 31 October. After getting discharge I submitted a claim, today they called me to say the claim was rejected because they don't cover mental health conditions. Is Mental health not a "serious? Also I took the cover back in 2022 because I'm a contractor so I don't get paid when I don't work. I just want an explanation as to why mental conditions aren't covered