Einstein was the first person to discover relativity, and how he stated those ideas is merely one way of countless different ways of expressing them. People learned what Einstein said, taught it to others and used those ideas in further development of physics. In the process they learned ways of expressing these ideas that are much more conducive to teaching novice physicists, and/or elegant to build upon. This is what I mean by better: faster learning, deeper understanding, mathematical and conceptual elegance.
This process is not unique to physics or Einstein. In most intellectual fields, people digest and reformulate the ideas of past intellectuals to make them better, at which point people stop reading the works of those past intellectuals (except historians or the super-experts), and start referring to the newer texts. In fact, if any field does not do this, I would dare call it an anti-intellectual field.
A field has certain goals, certain questions it tries to answer. In most intellectual fields, newcomers start by learning the best possible answers to those questions found till now, and then get to the research stage, where they start trying to do better. If they do find better answers, they teach the next generation those better answers.
If you are telling me that what Plato said is among the best answers to the questions that Philosophy asks, then that means the field has not progressed. You say that "short introductions ... misses out on the intricacies of that thought", but this indicates a deep problem with Philosophy. In every other field, experts of today express the ideas of those in the past with mistakes corrected, oversights fixed, and exposition improved. You also misunderstand me when you say 'rewriting and "simplifying" texts is the hallmark of progress.' Modern textbooks of General Relativity are far more intricate, and complex than what Einstein ever wrote, yet more understandable to the physics undergrad. Our answers today are far far better than Einstein's, so I won't waste an undergrad's time with Einstein's writings.
Where are the best answers that the field of Philosophy has come up with, and why were they not written in the past few decades?