It is just an opinion. We all have our quirks. This is one of mine. A wise nutritionist once told me not to eat anything my great grandmother would not recognize as food. I don't. I try to avoid most processed foods, and if soy milk isn't processed I don't know what is.
Part of my prejudice is based on past experience with soy based milk replacers. I am not sure they even make them any more but they used to. I learned if you feed them to a calf, the calf's problems will soon be over. Permanently. They can't digest the stuff. Now humans can digest it, I think, but I prefer the real stuff, not substitutes. Soy milk is a substitute for milk. The only possible reason I can think of for using it at all is because of a lactose intolerance or an allergy to milk. But that's me. If other people want to use it, fine. I'm just not convinced it is good for a person. That's my story and I'm sticking to it.