Wands. Wands are the reason. In the fictional universe of Harry Potter, magic is a fundamental force of nature. It is complex, wild, and untamed. It doesn’t respond to cute, Latinized incantations and pretty runes are written on paper. A small fraction of humans is born with the ability to control this force with varying degrees of strength. Prior to the invention of the wand, the use of magic required exceptional intellect, focus, and practice. It was haphazard and imprecise and frequently went terribly wrong. Wizards and witches had to control the magic consciously, using their sheer willpower to make it do what they wanted it to do. Then wands were invented. Wands made magic easier to use, and over time wizards discovered that once a magical effect was keyed to a spoken incantation, wands were able to produce that magical effect reliably and consistently with much less mental effort on the part of the caster. In this answer, I detail my theory as to why that happens. Modern witches and wizards are, for lack of a better word, spoiled by their wands. Frankly, they are magically lazy. The vast majority of them are perfectly happy restricting their magical education to learning the various useful wand spells created by better wizards. Very few wizards experiment with magic as a raw force of nature. It’s difficult to do so, not to mention dangerous, and by the time of the Harry Potter stories, there is a wand spell for almost every conceivable use. Much as modern youth are spoiled by smartphones and Google, modern witches and wizards are spoiled by their wands. There isn’t much motivation to learn about the vagaries and complexities of the fundamental force of magic. Their wands work just fine, thanks. Those ancient wizards? They had to be creative. They had to invent the spells. They had to invest years of their lives into mastering magic as a fundamental force, understanding it in ways that modern wizards would find completely unnecessary for living a normal life. Sure, they had wands, but far fewer incanted spells had already been invented. Many more magical tasks had to be accomplished the “old fashioned way,” even though yes, wands make that kind of magic a little easier to use, too. Think about the difference between an everyday teenager and Steve Jobs. Steve Jobs had the motivation and brains to change the world by doing what had never been done. The everyday teenager doesn’t care about changing the world, all they care about is that their iPhone works. They don’t care how it works, or why, and they sure as well couldn’t fix it if it broke. That’s why wizards like Dumbledore, Voldemort, and Grindelwald are becoming rarer and rarer. Do you think Voldemort found an incantation in a book that let him create a magical rock door that would only open if you put blood on it? No. He willed that enchantment into existence through sheer willpower, focus, and a deep understanding of the laws of magic. And some magic is beyond even the understanding and power of witches and wizards. The Sacrifice Charm that Lily inadvertently placed on Harry when she gave her life for him wasn’t a human creation. It wasn’t created by some ancient wizards. It was an expression of the deepest, fundamental laws of magic. It was pure nature, reacting to Lily’s selflessness and placing protection around Harry that no mortal wizard could hope to understand, much less undo. Dumbledore was able to tap into that powerful protection and, using the blood relation between Lily Potter and Petunia Dursley, forge a link that expanded that protection to Petunia’s home and the surrounding area. Do you think Dumbledore uttered a goofy Latin phrase and waved his wand to do that? No. It was a wordless expression of his willpower impressed onto the force of magic, an act of power that Dumbledore knew how to do because he was one of the few still alive that was so driven by an intellectual curiosity that he had to explore the deepest mysteries of magic. The average person lacks that drive. And so, because of wands, the average wizard is much less knowledgeable and capable of wielding magic itself than before. They simply don’t understand how it works anymore, apart from “this incantation produces this effect!” However, the average wizard is also much more powerful and capable of using magic — via wand spells — than the average wizard from the ancient world. They are just limited to the framework of existing spells, rather than needing to become adept at creating their own like more ancient wizards and witches. If a typically-skilled modern wizard-like Arthur Weasley went back in time to face Merlin, I think Merlin would be hard-pressed to defeat him. Mr. Weasley would have access to a dizzying array of easily-produced, fairly powerful offensive spells, while Merlin would be forced to wield raw magic against him. But that makes sense. Send any one of us back a thousand years equipped with a smartphone and some modern firearms, and we could do some serious damage to even the most skilled, legendary warriors of the day.