It's because there's a large gap in skill. I know people who play quite a bit and do nothing but pet battles, WQ's, transmog/mount farming and when it comes to raiding/m+/PvP, they are utter garbage. I like them as a person, but I wouldn't dare do raids/m+ with them as I'd get upset and have a bad time. I personally don't think any class in this game is hard to play, or atleast parse 80% on warcraftlogs. So with that mentality, I just get fed up with players when they can't perform optimally.
That being said, making the game harder for everyone might be okay for those who already play at a high level, but it would be horrible for those who don't even play their class right. People who aren't that good at the game might feel like the game isn't for them and players quitting is never a good thing for blizzard. As much as I'd like the open world to be more trivial and challenging, not every player out there shares that same view. Some people are perfectly content playing 1/2 their class and still being able to do WQ's.
In the end, it comes down to how people approach games. As a multisport athlete, my mentality was to compete and try my best. I carry that philosophy to video games and it's why I'm usually top 3 DPS in most raids and top dps in most dungeons I do. Knowing my rotation perfectly, knowing when to pop my CD's depending on the mobs, knowing when to hold off popping my CD's if I notice another DPS is going ham etc. Even when I'm top of the meters, I'm still looking for ways to improve and perform better. That's what makes the game fun for me because I look at it as a challenge to improve upon myself whenever I can. Not everyone shares that same ideology, some just want to casually push buttons and enjoy the process.
To each their own.