Because they get away with not doing it. Much easier. Plus if therapists gave true informed consent, itemizing the serious risks, explaining what is involved, giving some data on their track record and outcomes, giving a true account of their own personal mental health issues and the degree to which they have addressed them… it would be bad for business. They also should be giving a meaningful explanation of their methods, but often do not, because attempting it would probably show how much most are improvising and have no method.