AFAIK, this is the reason for this - Plato established the academy, so his school became quite dominant. And since his works were effectively the textbook, they

AFAIK, this is the reason for this - Plato established the academy, so his school became quite dominant. And since his works were effectively the textbook, they survived. After Plato, everyone's positions were defined relative to Plato's school, so it's easy to think that it's all based on Plato (and his teacher Socrates, and his student Aristotle). It's like saying all Chinese philosophy is based on Confucius, (and to a lessor extent Daoism, and Chinese Buddhism). They became the official schools, so everyone thought of other philosophers as being relative to Confucius.