It's not just that players got better, it's that we have vast amounts more information on literally anything accessible to us. There are guides, websites, discords, simulators, ect. We can analyse and figure out anything nowadays. WoW Classic experienced this recently, people remembered it as punishing and tough, but it was figured out in days and apparently felt quite simple.
Raid encounters have had to evolve to compensate. In 2003 you could get away with giving a boss 2-3 mechanics and it would feel difficult. You wouldn't have a well researched rotation and your gear would be random stuff you picked up along the way.
Nowadays on week 1 you have videos with every mechanic explained in detail, the optimal tricks and strats figured out. The optimal rotations are all figured out too, mathematically calculated and tested. Guides are abundant and any random person in your FC, linkshell or forum can tell you the indepth workings of your class, stats and tips.
This is why boss mechanics look like a small novel in modern MMO's, where you literally had 3 basic attacks 15 years ago. The only way they can really challenge us now is sensory overload.