Jeffrey Young developed schema therapy from a practical observation: a subset of patients weren't responding to standard CBT. Not because the method was wrong, but because their problem wasn't a recent thought — it was a forty-year-old pattern.
A schema is a model of yourself and others formed early, when basic needs went unmet. "Abandonment" — whoever gets close will leave. "Defectiveness" — if they really knew me, they'd reject me. "Self-sacrifice" — my needs come after everyone else's. About eighteen are described, and most people recognise themselves in two or three.
Schemas aren't thoughts you can contradict with evidence, because they weren't learned through logic. They were learned through repetition, in childhood, before there was language for them. Hence the need for different tools.