[unpopular opinion inbound]
Honestly?
I can give you an answer, but Y'all probably not like it.
It mainly due to the fact that you are in a group chat and a classroom. Given that the majority of the classroom/chat has not struggled with mental health issues, it is very easy to alienate people for the sake of being more popular. In fact, you yourself have probably done something similar at an unconscious level. It's not about physical/mental issues being more or less visible, or stupidity, it's about deliberate nastiness for the sake of popularity. And everyone is to blame for it.