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?
Not so in philosophy. If philosophy has the aim (if one's idea is that science has some goal, or a goal outside of being done for the sake of being done) to create new answers to old questions, or create new questions which require new answers, one must know (1) what questions were previously asked, (2) why the questions do (or don't) have answers, or what the problems and advantages of the answers are, (3) what these answers were in their original form, such that we have the best access to the most complete argument.
Endless rephrasing of texts didn't get us from Newton to Einstein. Even if one accepts that philosophy makes progress (internally), it doesn't in any way imply that one can get away without reading both old (to see what remains uncovered, and there is plenty to this day) and new.
No other field is philosophy. Philosophy, in some respects, is special - at least according to Socrates as relayed by Plato. As Socrates says, each science (or even activity) has its object, but what of the science which has science as its object? The trouble with many future reformulations of key ideas is that they can miss the intricacies and highly abstract arguments of the original texts. For that reason, it may be more appropriate to read Newton than it is to read Einstein, because Newton used much more natural language than equations. I would very heavily disagree with the idea that introductory textbooks necessarily have better contents and presentation than original texts, and any serious philosopher will tell you that too.
>If you are telling me that in the twenty centuries, no one has taken what Pythagoras/Plato said and has rewritten it in a better way, with modern exposition and examples and graphics and what not, I fail to believe you.
What do you mean by "a better way"? Is Plato really so difficult or obtuse or obscure that one needs to express Socrates' ideas in "a better way"? The text is perfectly readable to a modern audience, which is precisely why starting with Plato's actual dialogues is the hallmark of any good university philosophy programme.
>Because that would mean philosophy as a discipline is not improving by building upon the works of those past
No; you have it backwards. Many philosophers have agreed with or disagreed with Plato, but the point of a text on Plato's ideas should represent what he wanted to represent in those ideas, not say "well, that's what Plato said, here's my idea now!". Philosophy (according to some accounts, at least) does progress, but it's dubious that rewriting and "simplifying" texts is the hallmark of progress.
You end with the assumption that books written since 2000 are necessarily better than Plato. I fail to see why this is the case. If you want an overview of philosophy since Plato, then by all means, only the most up-to-date book will do. If you want to know Plato's (or Socrates') ideas yourself, there is no better source than Plato himself.
Philosophy explanations and short introductions (Peter Singer's one on Marx comes to mind) are riddled with errors, and for any such text you will find at least ten philosophers ready to lambast an "introductory" interpretation because it misses out on the intricacies of that thought. I'm not joking. Look at any intro book on Hegel, Kant, Marx, Pythagoras. Then look at the reviews published in philosophy journals.
"Progress" in philosophy does not and cannot mean shortening, rewriting and "simplifying" ideas.