system
Given the following post, respond as if you're commenting on Reddit:
human
[Fantasy] Why is magic almost always growing "weaker"?

It seems that in most fantasy stories, magic grows weaker as time goes on. What's the reason for this? 
Some being:

Harry Potter - The average wizard of today is nothing compared to the wizards of the past, even Voldemort and Dumbledore's magnificent fight is a pale imitation of what wizards were once capable of

Star Wars - The age of the great wars between the Jedi and Sith are long gone. Where there were once great battlefields with lightsaber being wielded and force powers being put to action are now merely men with blasters that don't even believe in the force blasting at each other, even the Clone Wars were nothing compared to the old wars, in both canon and legends

Lord of the Rings - The War of the Ring, though magical and fantastic by our standards is nothing compared to what once was. The elves are far weaker, Sauron and his ringwraiths pale in comparison to Morgoth and his balrogs, even the great and mighty Smaug is nothing compared to the dragons of old. 

The only exception I can think of is Avatar, where bending has grown more powerful.

Why is this? Why is magic often growing weaker?
gpt
In some universes, it's probably selection bias. People remember Merlin and Godric Gryffindor, and assume that all wizards of old must have been that powerful, but they forget all about Merlin's bumbling apprentice. Also, a story grows in the telling.