After seeing what Euclid's hit book series, bestseller of the historic world, was all about, I feel like it only makes sense that his ideas are still studied today. A lot of the ideas that he compiled are essential to the foundations of geometry, and I feel like a lot of it would coincide with mathematicians from different eras and civilizations, much like how a bat and a bird both develop wings but have never influenced each other in their evolutionary paths. After all, Euclid's book series included simple things like the properties of a circle, number theory, basic properties of various simple everyday shapes, and irrational numbers. The ancient Babylonians had something similar to the irrational numbers with their quadratic solving ideas. The ancient Egyptians also did a lot of work with geometry as we've seen from past readings on measurement, although it could be that Euclid's ideas were responsible for a lot of ancient Egyptian mathematics given that he was spending a lot of time there. With modern mathematics education, we see these topics arise all the time. We've seen the practical aspects of these geometries in fields like architecture, astronomy, computer science (particularly cryptography, where number theory is vital), and more, so it stands to reason that we would continue studying the foundations that have uplifted civilizations for centuries. We should continue to take these ideas and proof methods and keep discovering new things about them.
As for why The Elements might've been so important and popular in the past, it's probably because there weren't late-stage capitalistic opportunists trying to monopolize the textbook industry and sapping the finances of poor, starving scholars. (I jest.) I theorize that back then, mathematics appeared as a subject of fun and wonder, and the ideas in the books provided a hot new perspective on how to approach the subject. Would you look at that -- proofs! Rigorous proofs, if I may add! Now, there's an interesting thing that I noticed was missing from the reading, and that's the mention of "inductive proofs". Doing some quick Google searching, I found that inductive proofs were done as far back as the ancient Greeks, but it wasn't as widespread and certainly wasn't formalized as a method until the latter half of the 2nd millennium. But I digress. There was also just a social dynamic at play here: The Elements was simple and easy to parse! Consider this: you are interested in learning about mathematics. Your only option for research is to sift through piles of archaic texts and sort it all out yourself. Utterly cumbersome. Then comes along Euclid, the man, myth, or legends, and he gives you the equivalent of a Khan Academy or Paul's Math Notes -- nice and easy to decipher, easily communicated, and sorted in a comprehensive manner that saves you weeks of mind-numbing scholarly work. No wonder this book series was so popular -- it was a huge convenience! The other reason I can hypothesize is that the subject matter is just interesting, agreeable (logical), and reputable. Mathematicians likely continued seeing successes with the books, and the ideas are practical, so it makes sense why it was so popular.
In terms of the Euclidean postulates, notions, and principles for proofs, there is certainly a beauty to them. The beauty is basically along the lines of taking a small particular and creating the ideas of a whole discipline through the recursive process of creating new ideas and using them to prove more ideas (which in a way is like induction). In this case, the simplicity of Euclid's postulates is beautiful on their own. Oh, we can draw unique lines between a unique pair of points. Cool. These lines can be straight. Also, we can draw circles. These postulates are so simple, and we have such simple axioms like "a = c and b = c, so a = b". Then somehow, through a process of logical parkour, we can safely create a whole series of books of proofs, which were beautiful and inspiring on their own. Not only was this series providing neat ideas about geometry, but it was teaching people how to create their own ideas. Why was I doing direct proofs the way I was during my classes? It could very well be because of Euclid's work. But I digress. Based on how I'm describing the beauty of Euclid's ideas, I would summarize beauty in this way: beauty is about working with the micro to create the macro. It is about going from simplicity to complexity in an easy, flowing manner. It is unforced and natural, and that's what Euclid's work exactly is.