Because most of the consumer base now is normies/tourists who want to be part of an in group no matter how bad the product might be or how bad any of them are at the actual game. It's just a different slice of tribalism that's been part of history for centuries that until recently was far more frequently found in sports.

In the last 3 decades gaming went from a niche outcast hobby to one of the largest entertainment formats in the world and multiple sides of the industry pushed hard to not only normalise it but pander to an ideal low denominator of difficulty in order to maximise engagement and money generation. It's why later WoW expansions are not only easier in terms general approach, the bosses literally call out their attacks now so even those without DBM can know something is happening, but require fewer players. OG raiding was 20 and 40 man raid groups. TBC moved that to 10 and 25. Flex then did away with that even more. While there might be more going on in encounters now vs Vanilla, TBC, and Wrath the fights are still piss easy in comparison because classes are not as important, a single player can't fuck up everything except on the "harder" difficulties like mythic which are meant to still provide a challenge, and even if people don't bother with that they can still source gear from multiple other sources making what used to be the endgame of WoW redundant.

One of the worst parts is Blizz pushed for this. They literally chose to make their own game move away from one of the original appeals from 20 years ago, that's how braindead Blizz is as a company now. Another worse part is that the current playerbase wants this because they no longer have the attention spans or effort to do a dungeon that lasts more than 20 minutes or a raid that lasts more than an hour.