How many homeless people have you spoken to? Of course everybody is different so treating someone like a person may look different depending on their circumstances, but the principle still applies, including towards homeless people with severe mental illnesses.
And also that’s just not my experience tbh. Both from speaking to homeless people on the streets, and also when I was in prison I met a lot of the typical types of people who would end up street homeless, including people who were street homeless prior to being incarcerated and people who were street homeless and mentally ill, and they’re still people. I managed to form good friendships with a lot of them. I don’t know, possibly you have genuinely tried to interact with homeless people and you’ve hit a brick wall, but in my own experience that’s just an assumption or first impression people have and not the reality if you just try talking to them for a few mins.